diff --git a/app/billing/backend/supplier_invoices.py b/app/billing/backend/supplier_invoices.py
index aa76d79..6fe80f3 100644
--- a/app/billing/backend/supplier_invoices.py
+++ b/app/billing/backend/supplier_invoices.py
@@ -9,7 +9,7 @@ from typing import List, Dict, Optional
 from datetime import datetime, date, timedelta
 from decimal import Decimal
 from pathlib import Path
-from app.core.database import execute_query, execute_insert, execute_update, execute_query_single
+from app.core.database import execute_query, execute_insert, execute_update, execute_query_single, table_has_column
 from app.core.config import settings
 from app.services.economic_service import get_economic_service
 from app.services.ollama_service import ollama_service
@@ -710,8 +710,29 @@ async def list_supplier_invoices(
             params.append(vendor_id)
 
         if sag_id:
-            query += " AND si.sag_id = %s"
-            params.append(sag_id)
+            if table_has_column("supplier_invoices", "sag_id"):
+                query += " AND si.sag_id = %s"
+                params.append(sag_id)
+            elif (
+                table_has_column("supplier_invoice_relations", "supplier_invoice_id")
+                and table_has_column("supplier_invoice_relations", "relation_type")
+                and table_has_column("supplier_invoice_relations", "relation_id")
+            ):
+                query += """
+                    AND EXISTS (
+                        SELECT 1
+                        FROM supplier_invoice_relations sir
+                        WHERE sir.supplier_invoice_id = si.id
+                          AND sir.relation_type = 'sag'
+                          AND sir.relation_id = %s
+                    )
+                """
+                params.append(sag_id)
+            else:
+                logger.warning(
+                    "⚠️ supplier invoice sag filter requested, but no schema link available (sag_id column/relation table missing)"
+                )
+                query += " AND 1 = 0"
         
         if overdue_only:
             query += " AND si.due_date < CURRENT_DATE AND si.paid_date IS NULL"