完璧に動作しています。

テスト	結果
確定取消 API	✅ is_confirmed: false, confirmed_at: null
USE トランザクション削除	✅ current_stock が 27.5→32 に復帰
引当再作成	✅ reserved_stock = 5.000 に復帰
追加した変更:

stock_service.py:81-93 — unconfirm_spreading(): USE削除→確定フラグリセット→引当再作成
fertilizer/views.py — unconfirm アクション（POST /api/fertilizer/plans/{id}/unconfirm/）
fertilizer/page.tsx — 一覧に「確定取消」ボタン（確定済み計画のみ表示）
FertilizerEditPage.tsx — 編集画面ヘッダーに「確定取消」ボタン + 在庫情報再取得

backend/apps/materials/stock_service.py

@@ -0,0 +1,100 @@
+from decimal import Decimal, InvalidOperation
+
+from django.db import transaction
+from django.utils import timezone
+
+from .models import StockTransaction
+
+
+@transaction.atomic
+def create_reserves_for_plan(plan):
+    """施肥計画の引当を全置換で作り直す。"""
+    StockTransaction.objects.filter(
+        fertilization_plan=plan,
+        transaction_type=StockTransaction.TransactionType.RESERVE,
+    ).delete()
+
+    if plan.is_confirmed:
+        return
+
+    occurred_on = (
+        plan.updated_at.date() if getattr(plan, 'updated_at', None) else timezone.localdate()
+    )
+
+    for entry in plan.entries.select_related('fertilizer__material'):
+        material = getattr(entry.fertilizer, 'material', None)
+        if material is None:
+            continue
+        StockTransaction.objects.create(
+            material=material,
+            transaction_type=StockTransaction.TransactionType.RESERVE,
+            quantity=entry.bags,
+            occurred_on=occurred_on,
+            note=f'施肥計画「{plan.name}」からの引当',
+            fertilization_plan=plan,
+        )
+
+
+@transaction.atomic
+def delete_reserves_for_plan(plan):
+    """施肥計画に紐づく引当のみ削除する。"""
+    StockTransaction.objects.filter(
+        fertilization_plan=plan,
+        transaction_type=StockTransaction.TransactionType.RESERVE,
+    ).delete()
+
+
+@transaction.atomic
+def confirm_spreading(plan, actual_entries):
+    """引当を使用実績へ変換して施肥計画を確定済みにする。"""
+    from apps.fertilizer.models import Fertilizer
+
+    delete_reserves_for_plan(plan)
+
+    for entry_data in actual_entries:
+        actual_bags = _to_decimal(entry_data.get('actual_bags'))
+        if actual_bags <= 0:
+            continue
+
+        fertilizer = (
+            Fertilizer.objects.select_related('material')
+            .filter(id=entry_data['fertilizer_id'])
+            .first()
+        )
+        if fertilizer is None or fertilizer.material is None:
+            continue
+
+        StockTransaction.objects.create(
+            material=fertilizer.material,
+            transaction_type=StockTransaction.TransactionType.USE,
+            quantity=actual_bags,
+            occurred_on=timezone.localdate(),
+            note=f'施肥計画「{plan.name}」散布確定',
+            fertilization_plan=plan,
+        )
+
+    plan.is_confirmed = True
+    plan.confirmed_at = timezone.now()
+    plan.save(update_fields=['is_confirmed', 'confirmed_at'])
+
+
+@transaction.atomic
+def unconfirm_spreading(plan):
+    """散布確定を取り消し、USE トランザクションを削除して引当を再作成する。"""
+    StockTransaction.objects.filter(
+        fertilization_plan=plan,
+        transaction_type=StockTransaction.TransactionType.USE,
+    ).delete()
+
+    plan.is_confirmed = False
+    plan.confirmed_at = None
+    plan.save(update_fields=['is_confirmed', 'confirmed_at'])
+
+    create_reserves_for_plan(plan)
+
+
+def _to_decimal(value):
+    try:
+        return Decimal(str(value))
+    except (InvalidOperation, TypeError, ValueError):
+        return Decimal('0')