Do not convert invoiced quantity if sale has not unit when computing credit amount

Closes #13715

modules/sale_credit_limit/party.py

@@ -44,9 +44,13 @@ class Party(metaclass=PoolMeta):
                                 continue
                             invoice = invoice_line.invoice
                             if invoice and invoice.move:
-                                quantity -= Uom.compute_qty(
-                                    invoice_line.unit, invoice_line.quantity,
-                                    line.unit, round=False)
+                                if line.unit:
+                                    quantity -= Uom.compute_qty(
+                                        invoice_line.unit or line.unit,
+                                        invoice_line.quantity,
+                                        line.unit, round=False)
+                                else:
+                                    quantity -= invoice_line.quantity
                         if quantity > 0:
                             amount += Currency.compute(
                                 sale.currency,