fix(bx): allow reimburse submit when invoice wallet balance is insufficient

Drop the hard balance check in debit_reimburse so occupancy can exceed uploaded invoices; balance may go negative until later top-ups.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_bx_invoice_ledger.py

@@ -39,3 +39,15 @@ def test_release_idempotent_guard():
     assert should_skip_release(has_debit=True, has_release=True) is True
     assert should_skip_release(has_debit=True, has_release=False) is False
     assert should_skip_release(has_debit=False, has_release=False) is True
+
+
+def test_summarize_allows_negative_balance():
+    """提交占用可超过已上传总额，余额可为负（补票后再回升）。"""
+    rows = [
+        {"entry_type": ENTRY_CREDIT_UPLOAD, "amount": Decimal("100")},
+        {"entry_type": ENTRY_DEBIT_REIMBURSE, "amount": Decimal("300")},
+    ]
+    s = summarize_entries(rows)
+    assert s["total_amount"] == Decimal("100")
+    assert s["used_amount"] == Decimal("300")
+    assert s["balance"] == Decimal("-200")