Are they returning the item for credit or replacement? If they just want credit, DON'T reference the PO number in the PO field. Referencing the PO has the effect you are describing. If you need to reference the PO, use a comment field. And if you are returning for credit, complete the invoice and date fields so that a credit is generated in AP. But of course, the AP clerk has to wrestle with the difference in credit asked for and credit given. Has nothing to do with invoice being open to vendor. It is ALL about using the PO number in the PO field........