Creating a debit memo in Purchase Order module for returned items on PO


We had an employee try something new, and entered a bunch of returns using the PO module. These are outstanding in Purchasing Clearing. The credits from the vendor were processed via A/P directly to COGS. I'd like to invoice these returns, and get them out of purchasing clearing, even though I'd have to write them off from the vendors account subsequently. However when I go into Purchase Order> Process Receipt of Invoice and attempt to create a debit memo, the PO with open returns shows nothing available to invoice. Any suggestions? Thanks.