Add individually agreed membership fees (contribution overrides)

Members are sometimes given a fee that deviates from the regular
schedule -- e.g. a reduced rate for students -- for a specific period.
Add a per-member "Beitrag" tab where such deviations can be recorded
with a month-granular date range (Ab/Bis), a mandatory reason, and
either a fixed annual amount or a percentage discount off whichever
base rate is in effect at the time.

Data model: ContributionData gets a contribution_overrides list, each
entry validated (month format, Bis >= Ab, non-overlapping ranges per
member, reason required) and CRUD'd through the repository
(record/update/delete/get_contribution_override), consistent with how
donations already work.

Integration: contribution_claims.py now computes each membership-fee
claim's amount month by month instead of a single rate for the whole
billing period, picking up whichever override (if any) covers each
individual month. That handles an override starting or ending mid
period correctly (e.g. a semiannual payer whose discount begins in
March) without changing behavior for members without overrides.
Already-created claims are never recalculated retroactively, matching
how changes to the global contribution rates already behave.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ccma/domain/contributions.py

@@ -25,6 +25,11 @@ DONATION_STATUS_LABELS = {
     "overallocated": "ÜBERZUGEORDNET",
 }
 
+CONTRIBUTION_OVERRIDE_KIND_LABELS = {
+    "amount": "BETRAG",
+    "percent": "PROZENT",
+}
+
 
 def decimal_value(value: Any, field_name: str = "Betrag") -> Decimal:
     text = str(value).strip().replace(",", ".")
@@ -148,6 +153,26 @@ def donation_status(data: ContributionData, donation: dict[str, Any]) -> str:
     return "open"
 
 
+def contribution_override_covers_month(override: dict[str, Any], month: str) -> bool:
+    valid_from = str(override.get("valid_from", ""))
+    valid_until = str(override.get("valid_until", "") or "")
+    if valid_from and month < valid_from:
+        return False
+    if valid_until and month > valid_until:
+        return False
+    return True
+
+
+def contribution_override_ranges_overlap(
+    valid_from: str, valid_until: str, other_from: str, other_until: str
+) -> bool:
+    """Month ranges ('YYYY-MM', empty valid_until/other_until = open-ended) overlap
+    if each range starts no later than the other one ends."""
+    starts_before_other_ends = not other_until or valid_from <= other_until
+    other_starts_before_ends = not valid_until or other_from <= valid_until
+    return starts_before_other_ends and other_starts_before_ends
+
+
 def claim_status(data: ContributionData, claim: dict[str, Any], *, today: date | None = None) -> str:
     if str(claim.get("status", "")) == "cancelled":
         return "cancelled"

src/ccma/domain/dates.py

@@ -78,6 +78,38 @@ def normalize_date_input(value: str, field_name: str) -> str:
     return parsed.isoformat() if parsed else ""
 
 
+def month_input_hint() -> str:
+    return "MM.YYYY" if system_date_pattern() == "%d.%m.%Y" else "YYYY-MM"
+
+
+def parse_month_input(value: str, field_name: str, *, allow_empty: bool = True) -> str:
+    """Parses a month-only value (no day) and normalizes it to 'YYYY-MM', which also
+    sorts and compares correctly as a plain string."""
+    text = value.strip()
+    if not text:
+        if allow_empty:
+            return ""
+        raise DateValidationError(f"{field_name} ist erforderlich.")
+    for pattern, expected in (("%m.%Y", r"\d{2}\.\d{4}"), ("%Y-%m", r"\d{4}-\d{2}")):
+        if not re.fullmatch(expected, text):
+            continue
+        try:
+            parsed = datetime.strptime(text, pattern)
+        except ValueError:
+            continue
+        return f"{parsed.year:04d}-{parsed.month:02d}"
+    raise DateValidationError(f"{field_name} muss ein gültiger Monat im Format {month_input_hint()} sein.")
+
+
+def format_month_for_display(value: str) -> str:
+    text = value.strip()
+    match = re.fullmatch(r"(\d{4})-(\d{2})", text)
+    if not match:
+        return text
+    year, month = match.groups()
+    return f"{month}.{year}" if system_date_pattern() == "%d.%m.%Y" else f"{year}-{month}"
+
+
 def format_date_for_display(value: str) -> str:
     text = value.strip()
     if not text:

src/ccma/domain/models.py

@@ -324,6 +324,7 @@ class ContributionData:
     allocations: list[dict[str, Any]] = field(default_factory=list)
     reminders: list[dict[str, Any]] = field(default_factory=list)
     donations: list[dict[str, Any]] = field(default_factory=list)
+    contribution_overrides: list[dict[str, Any]] = field(default_factory=list)
     schema_version: int = 1
 
     def to_dict(self) -> dict[str, Any]:
@@ -335,6 +336,7 @@ class ContributionData:
             "allocations": self.allocations,
             "reminders": self.reminders,
             "donations": self.donations,
+            "contribution_overrides": self.contribution_overrides,
         }
 
     @classmethod
@@ -347,6 +349,7 @@ class ContributionData:
             allocations=list(data.get("allocations") or []),
             reminders=list(data.get("reminders") or []),
             donations=list(data.get("donations") or []),
+            contribution_overrides=list(data.get("contribution_overrides") or []),
         )
 
 

src/ccma/rules/scripts/contribution_claims.py

@@ -2,6 +2,7 @@ import calendar
 from datetime import date, timedelta
 from decimal import ROUND_HALF_UP, Decimal
 
+from ccma.domain.contributions import contribution_override_covers_month
 from ccma.domain.dates import DateValidationError, parse_iso_date
 from ccma.rules.api import RuleContext, create_claim
 from ccma.rules.scripts._shared import CONTRIBUTION_STATUSES
@@ -124,7 +125,7 @@ def _membership_claims(
         periods = [("annual", 1, 12, _due_date(year, rule.get("annual_due"), "01-31"))]
 
     actions = []
-    monthly_amount = annual_amount / Decimal(12)
+    overrides = context.contributions.contribution_overrides
     for suffix, first_month, last_month, regular_due in periods:
         # The entry year is intentionally billed from the entry month onward,
         # even when retroactive claims create old membership-fee claims.
@@ -132,7 +133,17 @@ def _membership_claims(
         months = max(0, last_month - charged_from + 1)
         if months == 0:
             continue
-        amount = (monthly_amount * months).quantize(CENT, rounding=ROUND_HALF_UP)
+        # Blended month by month rather than a single rate for the whole period, so a
+        # contribution override that starts or ends mid-period (e.g. a student discount
+        # beginning in March for a member who pays semiannually) is billed correctly.
+        month_amounts = [
+            _monthly_amount(context.repository_config, overrides, year, month)
+            for month in range(charged_from, last_month + 1)
+        ]
+        amount = sum((value for value, _override_id in month_amounts), Decimal("0")).quantize(
+            CENT, rounding=ROUND_HALF_UP
+        )
+        applied_overrides = sorted({override_id for _value, override_id in month_amounts if override_id})
         entry_year = started_at.year == year
         issue_date = regular_due - timedelta(days=issue_days)
         if not entry_year and context.today < issue_date:
@@ -168,6 +179,7 @@ def _membership_claims(
                         "annual_amount": _money(annual_amount),
                         "months": months,
                         "formula": "annual_amount * months / 12",
+                        "contribution_override_ids": applied_overrides,
                     },
                 },
             )
@@ -175,6 +187,28 @@ def _membership_claims(
     return actions
 
 
+def _monthly_amount(config, overrides, year: int, month: int) -> tuple[Decimal, str]:
+    """Returns the effective monthly rate for a single calendar month, and the
+    override's id if one applied (empty string otherwise)."""
+    rule = _rule_for(config, date(year, month, 1))
+    base_annual = Decimal(str(rule.get("annual_amount", "0"))) if rule else Decimal("0")
+    month_key = f"{year:04d}-{month:02d}"
+    override = next(
+        (item for item in overrides if contribution_override_covers_month(item, month_key)), None
+    )
+    if override is None:
+        return base_annual / Decimal(12), ""
+    kind = str(override.get("kind", ""))
+    value = Decimal(str(override.get("value", "0")))
+    if kind == "amount":
+        annual = value
+    elif kind == "percent":
+        annual = base_annual * (Decimal(100) - value) / Decimal(100)
+    else:
+        annual = base_annual
+    return annual / Decimal(12), str(override.get("override_id", ""))
+
+
 def _rule_for(config, target: date):
     selected = None
     for rule in config.get("contribution_rules", []):

src/ccma/storage/repository.py

@@ -14,9 +14,11 @@ from string import Formatter
 from uuid import uuid4
 
 from ccma.domain.contributions import (
+    CONTRIBUTION_OVERRIDE_KIND_LABELS,
     allocated_total,
     claim_balance,
     claim_total,
+    contribution_override_ranges_overlap,
     credit_allocated_total,
     decimal_value,
     donation_balance,
@@ -24,7 +26,12 @@ from ccma.domain.contributions import (
     money_text,
     payment_allocated_total,
 )
-from ccma.domain.dates import DateValidationError, normalize_date_input, validate_member_dates
+from ccma.domain.dates import (
+    DateValidationError,
+    normalize_date_input,
+    parse_month_input,
+    validate_member_dates,
+)
 from ccma.domain.models import (
     ASSET_CUSTODY_TYPE_LABELS,
     ASSET_OWNER_TYPE_LABELS,
@@ -811,7 +818,15 @@ class MemberRepository:
             raw = read_json(path)
             if not isinstance(raw, dict):
                 raise TypeError("Wurzelelement muss ein JSON-Objekt sein")
-            for field_name in ("claims", "payments", "credits", "allocations", "reminders"):
+            for field_name in (
+                "claims",
+                "payments",
+                "credits",
+                "allocations",
+                "reminders",
+                "donations",
+                "contribution_overrides",
+            ):
                 if field_name in raw and not isinstance(raw[field_name], list):
                     raise TypeError(f"{field_name} muss eine JSON-Liste sein")
                 if field_name in raw and any(not isinstance(item, dict) for item in raw[field_name]):
@@ -1786,6 +1801,171 @@ class MemberRepository:
         )
         return allocation
 
+    def get_contribution_override(self, member_id: str, override_id: str) -> tuple[ContributionData, dict]:
+        data = self.get_contributions(member_id)
+        override = next(
+            (
+                item
+                for item in data.contribution_overrides
+                if str(item.get("override_id", "")) == override_id
+            ),
+            None,
+        )
+        if override is None:
+            raise RepositoryError(f"Abweichender Beitrag nicht gefunden: {override_id}")
+        return data, override
+
+    def record_contribution_override(
+        self,
+        member_id: str,
+        *,
+        valid_from: str,
+        valid_until: str = "",
+        kind: str,
+        value: str,
+        reason: str,
+        actor_name: str = "Vorstand",
+    ) -> dict:
+        self.get_member(member_id)
+        normalized = self._normalize_contribution_override_input(
+            valid_from=valid_from, valid_until=valid_until, kind=kind, value=value, reason=reason
+        )
+        normalized_from, normalized_until, normalized_kind, normalized_value, normalized_reason = normalized
+        data = self.get_contributions(member_id)
+        self._assert_contribution_override_range_free(data, normalized_from, normalized_until)
+        override = {
+            "override_id": str(uuid4()),
+            "valid_from": normalized_from,
+            "valid_until": normalized_until,
+            "kind": normalized_kind,
+            "value": normalized_value,
+            "reason": normalized_reason,
+            "created_at": datetime.now().astimezone().isoformat(timespec="seconds"),
+            "created_by": actor_name,
+        }
+        data.contribution_overrides.append(override)
+        self.save_contributions(member_id, data)
+        self.append_event(
+            member_id,
+            event_type="contribution_override_recorded",
+            summary=f"Abweichender Mitgliedsbeitrag erfasst: {_contribution_override_summary(override)}",
+            actor_type="user",
+            actor_name=actor_name,
+            references={"override_id": override["override_id"]},
+        )
+        return override
+
+    def update_contribution_override(
+        self,
+        member_id: str,
+        override_id: str,
+        *,
+        valid_from: str,
+        valid_until: str = "",
+        kind: str,
+        value: str,
+        reason: str,
+        actor_name: str = "Vorstand",
+    ) -> dict:
+        normalized = self._normalize_contribution_override_input(
+            valid_from=valid_from, valid_until=valid_until, kind=kind, value=value, reason=reason
+        )
+        normalized_from, normalized_until, normalized_kind, normalized_value, normalized_reason = normalized
+        data, override = self.get_contribution_override(member_id, override_id)
+        self._assert_contribution_override_range_free(
+            data, normalized_from, normalized_until, exclude_override_id=override_id
+        )
+        override.update(
+            {
+                "valid_from": normalized_from,
+                "valid_until": normalized_until,
+                "kind": normalized_kind,
+                "value": normalized_value,
+                "reason": normalized_reason,
+                "updated_at": datetime.now().astimezone().isoformat(timespec="seconds"),
+            }
+        )
+        self.save_contributions(member_id, data)
+        self.append_event(
+            member_id,
+            event_type="contribution_override_changed",
+            summary=f"Abweichender Mitgliedsbeitrag geändert: {_contribution_override_summary(override)}",
+            actor_type="user",
+            actor_name=actor_name,
+            references={"override_id": override_id},
+        )
+        return override
+
+    def delete_contribution_override(
+        self, member_id: str, override_id: str, *, actor_name: str = "Vorstand"
+    ) -> None:
+        data, override = self.get_contribution_override(member_id, override_id)
+        data.contribution_overrides = [
+            item
+            for item in data.contribution_overrides
+            if str(item.get("override_id", "")) != override_id
+        ]
+        self.save_contributions(member_id, data)
+        self.append_event(
+            member_id,
+            event_type="contribution_override_deleted",
+            summary=f"Abweichender Mitgliedsbeitrag gelöscht: {_contribution_override_summary(override)}",
+            actor_type="user",
+            actor_name=actor_name,
+            references={"override_id": override_id},
+        )
+
+    @staticmethod
+    def _normalize_contribution_override_input(
+        *, valid_from: str, valid_until: str, kind: str, value: str, reason: str
+    ) -> tuple[str, str, str, str, str]:
+        try:
+            normalized_from = parse_month_input(valid_from, "Ab", allow_empty=False)
+            normalized_until = parse_month_input(valid_until, "Bis")
+        except DateValidationError as exc:
+            raise RepositoryError(str(exc)) from exc
+        if normalized_until and normalized_until < normalized_from:
+            raise RepositoryError("Der Zeitraum 'Bis' darf nicht vor dem Zeitraum 'Ab' liegen.")
+        normalized_kind = kind.strip().lower()
+        if normalized_kind not in CONTRIBUTION_OVERRIDE_KIND_LABELS:
+            raise RepositoryError("Ungültige Art des abweichenden Beitrags.")
+        try:
+            parsed_value = decimal_value(value, "Wert")
+        except ValueError as exc:
+            raise RepositoryError(str(exc)) from exc
+        if normalized_kind == "percent":
+            if parsed_value <= 0 or parsed_value > 100:
+                raise RepositoryError("Die Ermäßigung muss zwischen 0 und 100 Prozent liegen.")
+        elif parsed_value < 0:
+            raise RepositoryError("Der Jahresbetrag darf nicht negativ sein.")
+        normalized_reason = reason.strip()
+        if not normalized_reason:
+            raise RepositoryError("Eine Begründung ist erforderlich.")
+        return normalized_from, normalized_until, normalized_kind, money_text(parsed_value), normalized_reason
+
+    @staticmethod
+    def _assert_contribution_override_range_free(
+        data: ContributionData,
+        valid_from: str,
+        valid_until: str,
+        *,
+        exclude_override_id: str | None = None,
+    ) -> None:
+        for existing in data.contribution_overrides:
+            if exclude_override_id and str(existing.get("override_id", "")) == exclude_override_id:
+                continue
+            if contribution_override_ranges_overlap(
+                valid_from,
+                valid_until,
+                str(existing.get("valid_from", "")),
+                str(existing.get("valid_until", "") or ""),
+            ):
+                existing_until = str(existing.get("valid_until", "") or "") or "unbefristet"
+                raise RepositoryError(
+                    "Der Zeitraum überschneidet sich mit einem bereits vorhandenen abweichenden "
+                    f"Beitrag ({existing.get('valid_from', '')} bis {existing_until})."
+                )
+
     def create_reminder_draft(
         self,
         member_id: str,
@@ -2455,6 +2635,15 @@ def _german_date(value: str) -> str:
         return ""
 
 
+def _contribution_override_summary(override: dict) -> str:
+    value = str(override.get("value", ""))
+    value_text = f"{value}%" if str(override.get("kind", "")) == "percent" else f"{value} EUR"
+    valid_from = str(override.get("valid_from", ""))
+    valid_until = str(override.get("valid_until", "") or "")
+    period = f"{valid_from} bis {valid_until}" if valid_until else f"ab {valid_from}"
+    return f"{value_text} ({period})"
+
+
 def _dunning_hold_is_active(claim: dict) -> bool:
     hold = claim.get("dunning_hold") or {}
     if not hold.get("active"):

src/ccma/ui/contribution_override_dialog.py

@@ -0,0 +1,139 @@
+from __future__ import annotations
+
+import tkinter as tk
+from collections.abc import Callable
+from tkinter import messagebox, ttk
+
+from ccma.domain.contributions import CONTRIBUTION_OVERRIDE_KIND_LABELS
+from ccma.domain.dates import format_month_for_display, month_input_hint
+from ccma.storage.repository import MemberRepository, RepositoryError
+from ccma.ui.labels import storage_key
+
+
+class _Dialog(tk.Toplevel):
+    def __init__(self, master: tk.Misc, title: str, on_saved: Callable[[], None]):
+        super().__init__(master)
+        self.on_saved = on_saved
+        self.title(title)
+        self.transient(master.winfo_toplevel())
+        self.resizable(False, False)
+        self.frame = ttk.Frame(self, padding=18)
+        self.frame.pack(fill="both", expand=True)
+        self.bind("<Escape>", lambda _event: self.destroy())
+        self.after_idle(self.grab_set)
+
+    def _buttons(self, row: int, command: Callable[[], None]) -> None:
+        buttons = ttk.Frame(self.frame)
+        buttons.grid(row=row, column=0, columnspan=2, sticky="e", pady=(16, 0))
+        ttk.Button(buttons, text="Abbrechen", command=self.destroy).pack(side="left", padx=(0, 8))
+        ttk.Button(buttons, text="Speichern", style="Accent.TButton", command=command).pack(side="left")
+
+
+class ContributionOverrideEditDialog(_Dialog):
+    """Records an individually agreed membership fee for a member -- e.g. a reduced
+    student rate -- for a month-granular date range, either as a fixed annual amount
+    or as a percentage discount off whichever base rate is in effect at the time."""
+
+    def __init__(
+        self,
+        master: tk.Misc,
+        repository: MemberRepository,
+        member_id: str,
+        on_saved: Callable[[], None],
+        override: dict | None = None,
+    ):
+        super().__init__(
+            master,
+            "Abweichenden Beitrag bearbeiten" if override else "Abweichenden Beitrag anlegen",
+            on_saved,
+        )
+        self.repository = repository
+        self.member_id = member_id
+        self.override_id = str(override.get("override_id")) if override else None
+        kind_label = CONTRIBUTION_OVERRIDE_KIND_LABELS.get(
+            str(override.get("kind", "")) if override else "amount",
+            CONTRIBUTION_OVERRIDE_KIND_LABELS["amount"],
+        )
+        self.variables = {
+            "valid_from": tk.StringVar(
+                value=format_month_for_display(str(override.get("valid_from", ""))) if override else ""
+            ),
+            "valid_until": tk.StringVar(
+                value=format_month_for_display(str(override.get("valid_until", ""))) if override else ""
+            ),
+            "kind": tk.StringVar(value=kind_label),
+            "value": tk.StringVar(value=str(override.get("value", "")) if override else ""),
+        }
+
+        self.frame.columnconfigure(1, weight=1)
+        ttk.Label(self.frame, text=f"Ab ({month_input_hint()})").grid(
+            row=0, column=0, sticky="w", pady=5, padx=(0, 12)
+        )
+        ttk.Entry(self.frame, textvariable=self.variables["valid_from"], width=42).grid(
+            row=0, column=1, sticky="ew", pady=5
+        )
+        ttk.Label(self.frame, text=f"Bis ({month_input_hint()}, leer = unbefristet)").grid(
+            row=1, column=0, sticky="w", pady=5, padx=(0, 12)
+        )
+        ttk.Entry(self.frame, textvariable=self.variables["valid_until"], width=42).grid(
+            row=1, column=1, sticky="ew", pady=5
+        )
+        ttk.Label(self.frame, text="Art").grid(row=2, column=0, sticky="w", pady=5, padx=(0, 12))
+        kind_combo = ttk.Combobox(
+            self.frame,
+            textvariable=self.variables["kind"],
+            values=list(CONTRIBUTION_OVERRIDE_KIND_LABELS.values()),
+            state="readonly",
+            width=39,
+        )
+        kind_combo.grid(row=2, column=1, sticky="ew", pady=5)
+        kind_combo.bind("<<ComboboxSelected>>", lambda _event: self._update_value_label())
+        self.value_label_var = tk.StringVar()
+        ttk.Label(self.frame, textvariable=self.value_label_var).grid(
+            row=3, column=0, sticky="w", pady=5, padx=(0, 12)
+        )
+        ttk.Entry(self.frame, textvariable=self.variables["value"], width=42).grid(
+            row=3, column=1, sticky="ew", pady=5
+        )
+        ttk.Label(self.frame, text="Begründung").grid(row=4, column=0, sticky="nw", pady=5, padx=(0, 12))
+        self.reason_text = tk.Text(self.frame, width=42, height=4, wrap="word")
+        self.reason_text.grid(row=4, column=1, sticky="ew", pady=5)
+        if override:
+            self.reason_text.insert("1.0", str(override.get("reason", "")))
+        self._update_value_label()
+        self._buttons(5, self._save)
+
+    def _update_value_label(self) -> None:
+        kind = storage_key(CONTRIBUTION_OVERRIDE_KIND_LABELS, self.variables["kind"].get())
+        self.value_label_var.set("Jahresbetrag (EUR)" if kind == "amount" else "Ermäßigung (%)")
+
+    def _save(self) -> None:
+        kind = storage_key(CONTRIBUTION_OVERRIDE_KIND_LABELS, self.variables["kind"].get())
+        reason = self.reason_text.get("1.0", "end-1c")
+        try:
+            if self.override_id:
+                self.repository.update_contribution_override(
+                    self.member_id,
+                    self.override_id,
+                    valid_from=self.variables["valid_from"].get(),
+                    valid_until=self.variables["valid_until"].get(),
+                    kind=kind,
+                    value=self.variables["value"].get(),
+                    reason=reason,
+                )
+            else:
+                self.repository.record_contribution_override(
+                    self.member_id,
+                    valid_from=self.variables["valid_from"].get(),
+                    valid_until=self.variables["valid_until"].get(),
+                    kind=kind,
+                    value=self.variables["value"].get(),
+                    reason=reason,
+                )
+        except RepositoryError as exc:
+            messagebox.showerror(
+                "Abweichender Beitrag konnte nicht gespeichert werden", str(exc), parent=self
+            )
+            return
+        self.destroy()
+        self.on_saved()

src/ccma/ui/member_tab.py

@@ -10,6 +10,7 @@ from tkinter import messagebox, ttk
 from ccma.config import AppConfig
 from ccma.domain.contributions import (
     CLAIM_STATUS_LABELS,
+    CONTRIBUTION_OVERRIDE_KIND_LABELS,
     DONATION_STATUS_LABELS,
     claim_status,
     claim_total,
@@ -19,10 +20,11 @@ from ccma.domain.contributions import (
     money_text,
     payment_allocated_total,
 )
-from ccma.domain.dates import age_label, date_input_hint, format_date_for_display
+from ccma.domain.dates import age_label, date_input_hint, format_date_for_display, format_month_for_display
 from ccma.domain.models import ASSET_STATUS_LABELS, PAYMENT_FREQUENCY_LABELS, Event
 from ccma.domain.models import MEMBERSHIP_STATUS_LABELS as STATUS_LABELS
 from ccma.storage.repository import MemberRepository, RepositoryError
+from ccma.ui.contribution_override_dialog import ContributionOverrideEditDialog
 from ccma.ui.dialogs import IntegrityWarningDialog
 from ccma.ui.document_dialog import DocumentTemplateDialog
 from ccma.ui.donation_dialog import (
@@ -174,11 +176,13 @@ class MemberTab(ttk.Frame):
             command=self._save,
         ).grid(row=0, column=0, sticky="e")
         contribution_tab = ttk.Frame(notebook, padding=16)
+        contribution_override_tab = ttk.Frame(notebook, padding=16)
         payments_tab = ttk.Frame(notebook, padding=16)
         donations_tab = ttk.Frame(notebook, padding=16)
         assets_tab = ttk.Frame(notebook, padding=16)
         documents_tab = ttk.Frame(notebook, padding=16)
         notebook.add(contribution_tab, text="Forderungen")
+        notebook.add(contribution_override_tab, text="Beitrag")
         notebook.add(payments_tab, text="Zahlungen")
         notebook.add(donations_tab, text="Spenden")
         notebook.add(assets_tab, text="Assets")
@@ -302,6 +306,58 @@ class MemberTab(ttk.Frame):
         self.claims.bind("<Double-1>", lambda _event: self._open_selected_claim())
         self.claims.bind("<Return>", lambda _event: self._open_selected_claim())
 
+        contribution_override_tab.columnconfigure(0, weight=1)
+        contribution_override_tab.rowconfigure(1, weight=1)
+        ttk.Label(
+            contribution_override_tab,
+            text=(
+                "Individuell vereinbarte Beiträge (z. B. ermäßigter Beitrag für Schüler), die von "
+                "den regulären Beitragssätzen abweichen. Wirkt sich erst auf künftig erzeugte "
+                "Forderungen aus, bereits erzeugte Forderungen bleiben unverändert."
+            ),
+            style="Mono.TLabel",
+            wraplength=700,
+        ).grid(row=0, column=0, sticky="w", pady=(0, 10))
+        self.contribution_overrides = ttk.Treeview(
+            contribution_override_tab,
+            columns=("valid_from", "valid_until", "kind", "value", "reason"),
+            show="headings",
+            selectmode="browse",
+        )
+        for key, title, width in (
+            ("valid_from", "Ab", FIXED_COLUMN_WIDTH),
+            ("valid_until", "Bis", FIXED_COLUMN_WIDTH),
+            ("kind", "Art", FIXED_COLUMN_WIDTH),
+            ("value", "Wert", FIXED_COLUMN_WIDTH),
+            ("reason", "Begründung", 320),
+        ):
+            self.contribution_overrides.heading(key, text=title)
+            self.contribution_overrides.column(key, width=width, anchor="w", stretch=key == "reason")
+        self.contribution_overrides.grid(row=1, column=0, sticky="nsew")
+        self.contribution_overrides.bind(
+            "<Double-1>", lambda _event: self._edit_selected_contribution_override()
+        )
+        self.contribution_overrides.bind(
+            "<Return>", lambda _event: self._edit_selected_contribution_override()
+        )
+        contribution_override_actions = ttk.Frame(contribution_override_tab)
+        contribution_override_actions.grid(row=2, column=0, sticky="e", pady=(8, 0))
+        ttk.Button(
+            contribution_override_actions,
+            text="Abweichenden Beitrag anlegen",
+            command=self._create_contribution_override,
+        ).pack(side="left", padx=(0, 8))
+        ttk.Button(
+            contribution_override_actions,
+            text="Bearbeiten",
+            command=self._edit_selected_contribution_override,
+        ).pack(side="left", padx=(0, 8))
+        ttk.Button(
+            contribution_override_actions,
+            text="Löschen",
+            command=self._delete_selected_contribution_override,
+        ).pack(side="left")
+
         payments_tab.columnconfigure(0, weight=1)
         payments_tab.rowconfigure(1, weight=1)
         self.payment_summary = tk.StringVar()
@@ -573,6 +629,7 @@ class MemberTab(ttk.Frame):
         self._clear_dirty()
         self._refresh_events()
         self._refresh_contributions()
+        self._refresh_contribution_overrides()
         self._refresh_donations()
         self._refresh_assets()
         self._refresh_documents()
@@ -686,6 +743,31 @@ class MemberTab(ttk.Frame):
         )
         self.donation_summary.set(f"{len(data.donations)} Spenden · Gesamt {money_text(total_amount)} EUR")
 
+    def _refresh_contribution_overrides(self) -> None:
+        self.contribution_overrides.delete(*self.contribution_overrides.get_children())
+        try:
+            data = self.repository.get_contributions(self.member_id)
+        except RepositoryError:
+            return
+        for override in sorted(
+            data.contribution_overrides, key=lambda item: str(item.get("valid_from", "")), reverse=True
+        ):
+            kind = str(override.get("kind", ""))
+            value = str(override.get("value", ""))
+            value_text = f"{value} %" if kind == "percent" else f"{value} EUR"
+            self.contribution_overrides.insert(
+                "",
+                "end",
+                iid=str(override.get("override_id", "")),
+                values=(
+                    format_month_for_display(str(override.get("valid_from", ""))),
+                    format_month_for_display(str(override.get("valid_until", ""))) or "unbefristet",
+                    CONTRIBUTION_OVERRIDE_KIND_LABELS.get(kind, kind.upper()),
+                    value_text,
+                    override.get("reason", ""),
+                ),
+            )
+
     def _toggle_claim_sort(self, column: str) -> None:
         if self.claim_sort_column == column:
             self.claim_sort_descending = not self.claim_sort_descending
@@ -834,6 +916,59 @@ class MemberTab(ttk.Frame):
         self.refresh()
         self.on_changed()
 
+    def _selected_contribution_override(self) -> dict | None:
+        selected = self.contribution_overrides.selection()
+        if not selected:
+            return None
+        try:
+            _data, override = self.repository.get_contribution_override(self.member_id, selected[0])
+        except RepositoryError:
+            return None
+        return override
+
+    def _create_contribution_override(self) -> None:
+        ContributionOverrideEditDialog(
+            self, self.repository, self.member_id, self._contribution_override_changed
+        )
+
+    def _edit_selected_contribution_override(self) -> None:
+        override = self._selected_contribution_override()
+        if not override:
+            messagebox.showinfo(
+                "Eintrag auswählen", "Bitte einen abweichenden Beitrag auswählen.", parent=self
+            )
+            return
+        ContributionOverrideEditDialog(
+            self, self.repository, self.member_id, self._contribution_override_changed, override
+        )
+
+    def _delete_selected_contribution_override(self) -> None:
+        override = self._selected_contribution_override()
+        if not override:
+            messagebox.showinfo(
+                "Eintrag auswählen", "Bitte einen abweichenden Beitrag auswählen.", parent=self
+            )
+            return
+        if not messagebox.askyesno(
+            "Abweichenden Beitrag löschen",
+            "Diesen abweichenden Beitrag wirklich löschen? Bereits erzeugte Forderungen bleiben "
+            "davon unberührt.",
+            parent=self,
+        ):
+            return
+        try:
+            self.repository.delete_contribution_override(
+                self.member_id, str(override.get("override_id", ""))
+            )
+        except RepositoryError as exc:
+            messagebox.showerror("Löschen fehlgeschlagen", str(exc), parent=self)
+            return
+        self._contribution_override_changed()
+
+    def _contribution_override_changed(self) -> None:
+        self.refresh()
+        self.on_changed()
+
     def _refresh_documents(self) -> None:
         self.documents.delete(*self.documents.get_children())
         self.document_paths.clear()

tests/test_contribution_overrides.py

@@ -0,0 +1,300 @@
+from datetime import date
+
+import pytest
+
+from ccma.services.housekeeper import Housekeeper
+from ccma.storage.repository import MemberRepository, RepositoryError
+
+
+def _member(tmp_path):
+    repository = MemberRepository(tmp_path)
+    repository.initialize()
+    member = repository.create_member(first_name="Override", last_name="Test", birth_date="2010-01-01")
+    member.status = "active"
+    member.accepted_at = "2026-01-01"
+    member.membership_started_at = "2026-01-01"
+    member.payment_frequency = "semiannual"
+    repository.save_member(member)
+    return repository, member
+
+
+def test_contribution_override_can_be_recorded_with_amount(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+
+    override = repository.record_contribution_override(
+        member.member_id,
+        valid_from="03.2026",
+        valid_until="",
+        kind="amount",
+        value="90.00",
+        reason="Schüler laut Nachweis",
+    )
+
+    data = repository.get_contributions(member.member_id)
+    assert data.contribution_overrides == [override]
+    assert override["valid_from"] == "2026-03"
+    assert override["valid_until"] == ""
+    assert override["kind"] == "amount"
+    assert override["value"] == "90.00"
+    assert override["reason"] == "Schüler laut Nachweis"
+    assert repository.get_events(member.member_id)[-1].event_type == "contribution_override_recorded"
+
+
+def test_contribution_override_accepts_percent_kind(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+
+    override = repository.record_contribution_override(
+        member.member_id,
+        valid_from="2026-03",
+        valid_until="2026-08",
+        kind="percent",
+        value="50",
+        reason="Schüler laut Nachweis",
+    )
+
+    assert override["kind"] == "percent"
+    assert override["value"] == "50.00"
+    assert override["valid_until"] == "2026-08"
+
+
+def test_contribution_override_requires_reason(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+
+    with pytest.raises(RepositoryError, match="Begründung"):
+        repository.record_contribution_override(
+            member.member_id, valid_from="03.2026", kind="amount", value="90.00", reason="   "
+        )
+
+
+def test_contribution_override_rejects_invalid_percent(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+
+    with pytest.raises(RepositoryError, match="zwischen 0 und 100"):
+        repository.record_contribution_override(
+            member.member_id, valid_from="03.2026", kind="percent", value="150", reason="Test"
+        )
+    with pytest.raises(RepositoryError, match="zwischen 0 und 100"):
+        repository.record_contribution_override(
+            member.member_id, valid_from="03.2026", kind="percent", value="0", reason="Test"
+        )
+
+
+def test_contribution_override_rejects_negative_amount(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+
+    with pytest.raises(RepositoryError, match="nicht negativ"):
+        repository.record_contribution_override(
+            member.member_id, valid_from="03.2026", kind="amount", value="-10", reason="Test"
+        )
+
+
+def test_contribution_override_rejects_until_before_from(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+
+    with pytest.raises(RepositoryError, match="darf nicht vor"):
+        repository.record_contribution_override(
+            member.member_id,
+            valid_from="08.2026",
+            valid_until="03.2026",
+            kind="amount",
+            value="90.00",
+            reason="Test",
+        )
+
+
+def test_overlapping_contribution_overrides_are_rejected(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+    repository.record_contribution_override(
+        member.member_id,
+        valid_from="2026-01",
+        valid_until="2026-06",
+        kind="amount",
+        value="90.00",
+        reason="Erstes",
+    )
+
+    with pytest.raises(RepositoryError, match="überschneidet sich"):
+        repository.record_contribution_override(
+            member.member_id,
+            valid_from="2026-04",
+            valid_until="2026-12",
+            kind="amount",
+            value="100.00",
+            reason="Zweites",
+        )
+
+    # Adjacent, non-overlapping range is fine.
+    second = repository.record_contribution_override(
+        member.member_id,
+        valid_from="2026-07",
+        valid_until="",
+        kind="amount",
+        value="100.00",
+        reason="Zweites, ab Juli",
+    )
+    assert second["valid_from"] == "2026-07"
+
+
+def test_open_ended_override_overlaps_with_any_later_range(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+    repository.record_contribution_override(
+        member.member_id, valid_from="2026-01", kind="amount", value="90.00", reason="Unbefristet"
+    )
+
+    with pytest.raises(RepositoryError, match="überschneidet sich"):
+        repository.record_contribution_override(
+            member.member_id,
+            valid_from="2030-01",
+            valid_until="2030-06",
+            kind="amount",
+            value="100.00",
+            reason="Später",
+        )
+
+
+def test_contribution_override_can_be_updated_and_deleted(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+    override = repository.record_contribution_override(
+        member.member_id, valid_from="2026-01", kind="amount", value="90.00", reason="Erstversion"
+    )
+
+    updated = repository.update_contribution_override(
+        member.member_id,
+        override["override_id"],
+        valid_from="2026-02",
+        valid_until="2026-12",
+        kind="percent",
+        value="40",
+        reason="Korrigiert",
+    )
+    assert updated["valid_from"] == "2026-02"
+    assert updated["kind"] == "percent"
+    assert updated["reason"] == "Korrigiert"
+    assert repository.get_events(member.member_id)[-1].event_type == "contribution_override_changed"
+
+    repository.delete_contribution_override(member.member_id, override["override_id"])
+    data = repository.get_contributions(member.member_id)
+    assert data.contribution_overrides == []
+    assert repository.get_events(member.member_id)[-1].event_type == "contribution_override_deleted"
+
+
+def test_updating_override_can_keep_its_own_range(tmp_path) -> None:
+    repository, member = _member(tmp_path)
+    override = repository.record_contribution_override(
+        member.member_id,
+        valid_from="2026-01",
+        valid_until="2026-06",
+        kind="amount",
+        value="90.00",
+        reason="Erstversion",
+    )
+
+    # Saving again with the same range must not trip the overlap check against itself.
+    updated = repository.update_contribution_override(
+        member.member_id,
+        override["override_id"],
+        valid_from="2026-01",
+        valid_until="2026-06",
+        kind="amount",
+        value="95.00",
+        reason="Betrag korrigiert",
+    )
+    assert updated["value"] == "95.00"
+
+
+def _housekeeper_member(tmp_path, *, payment_frequency="annual", started_at="2025-01-01"):
+    repository = MemberRepository(tmp_path)
+    repository.initialize()
+    member = repository.create_member(first_name="Override", last_name="Claims", birth_date="1990-01-01")
+    member.status = "active"
+    member.accepted_at = started_at
+    member.membership_started_at = started_at
+    member.payment_frequency = payment_frequency
+    repository.save_member(member)
+    return repository, member
+
+
+def test_full_year_amount_override_replaces_the_annual_claim_amount(tmp_path) -> None:
+    repository, member = _housekeeper_member(tmp_path, payment_frequency="annual")
+    repository.record_contribution_override(
+        member.member_id,
+        valid_from="2026-01",
+        valid_until="2026-12",
+        kind="amount",
+        value="60.00",
+        reason="Schüler laut Nachweis",
+    )
+
+    Housekeeper(repository).run(today=date(2026, 6, 21))
+
+    claims_by_key = {
+        claim["claim_key"]: claim for claim in repository.get_contributions(member.member_id).claims
+    }
+    claim = claims_by_key["membership-fee:2026:annual"]
+    assert claim["amount"] == "60.00"
+    assert claim["calculation"]["contribution_override_ids"] != []
+
+
+def test_full_year_percent_override_reduces_the_base_rate(tmp_path) -> None:
+    repository, member = _housekeeper_member(tmp_path, payment_frequency="annual")
+    repository.record_contribution_override(
+        member.member_id,
+        valid_from="2026-01",
+        valid_until="2026-12",
+        kind="percent",
+        value="50",
+        reason="Schüler laut Nachweis",
+    )
+
+    Housekeeper(repository).run(today=date(2026, 6, 21))
+
+    claim = next(
+        claim
+        for claim in repository.get_contributions(member.member_id).claims
+        if claim["claim_key"] == "membership-fee:2026:annual"
+    )
+    assert claim["amount"] == "75.00"
+
+
+def test_override_starting_mid_period_blends_month_by_month(tmp_path) -> None:
+    repository, member = _housekeeper_member(tmp_path, payment_frequency="semiannual")
+    repository.record_contribution_override(
+        member.member_id,
+        valid_from="2026-03",
+        valid_until="2026-08",
+        kind="amount",
+        value="60.00",
+        reason="Schüler ab März",
+    )
+
+    Housekeeper(repository).run(today=date(2026, 12, 31))
+
+    claims_by_key = {
+        claim["claim_key"]: claim for claim in repository.get_contributions(member.member_id).claims
+    }
+    # Jan+Feb at 12.50/month (full rate) + Mar-Jun at 5.00/month (override) = 45.00
+    assert claims_by_key["membership-fee:2026:first-half"]["amount"] == "45.00"
+    # Jul+Aug at 5.00/month (override) + Sep-Dec at 12.50/month (full rate) = 60.00
+    assert claims_by_key["membership-fee:2026:second-half"]["amount"] == "60.00"
+
+
+def test_claim_without_applicable_override_is_unaffected(tmp_path) -> None:
+    repository, member = _housekeeper_member(tmp_path, payment_frequency="annual")
+    repository.record_contribution_override(
+        member.member_id,
+        valid_from="2020-01",
+        valid_until="2020-12",
+        kind="amount",
+        value="60.00",
+        reason="Frueherer Zeitraum",
+    )
+
+    Housekeeper(repository).run(today=date(2026, 6, 21))
+
+    claim = next(
+        claim
+        for claim in repository.get_contributions(member.member_id).claims
+        if claim["claim_key"] == "membership-fee:2026:annual"
+    )
+    assert claim["amount"] == "150.00"
+    assert claim["calculation"]["contribution_override_ids"] == []

tests/test_dates.py

@@ -7,9 +7,11 @@ from ccma.domain.dates import (
     age_label,
     calculate_age,
     format_date_for_display,
+    format_month_for_display,
     normalize_date_input,
     parse_date_input,
     parse_iso_date,
+    parse_month_input,
     validate_birth_date,
     validate_member_dates,
 )
@@ -68,6 +70,25 @@ def test_member_dates_must_be_chronological() -> None:
         )
 
 
+def test_month_input_accepts_german_and_iso_formats() -> None:
+    assert parse_month_input("03.2026", "Ab") == "2026-03"
+    assert parse_month_input("2026-03", "Ab") == "2026-03"
+    assert parse_month_input("", "Ab") == ""
+    with pytest.raises(DateValidationError, match="erforderlich"):
+        parse_month_input("", "Ab", allow_empty=False)
+    with pytest.raises(DateValidationError):
+        parse_month_input("13.2026", "Ab")
+    with pytest.raises(DateValidationError):
+        parse_month_input("2026-03-15", "Ab")
+
+
+def test_month_display_uses_system_pattern(monkeypatch) -> None:
+    monkeypatch.setattr("ccma.domain.dates.system_date_pattern", lambda: "%d.%m.%Y")
+    assert format_month_for_display("2026-03") == "03.2026"
+    monkeypatch.setattr("ccma.domain.dates.system_date_pattern", lambda: "%Y-%m-%d")
+    assert format_month_for_display("2026-03") == "2026-03"
+
+
 def test_age_calculation_and_label() -> None:
     today = date(2026, 6, 21)
     assert calculate_age(date(2000, 6, 21), today) == 26