We were trying to enter a received PO as an invoice that had two line items received ( and two on the invoice), one for a purchased part and one for a return for credit.  The balance on the PO receipt for that PO was negative.  When we entered the negative total in PO-C as the invoice total, only the negative receipt could be chosen, and that didn't work because the total for the line item was larger( more negative) than the total for the invoice. The positive receipt could not be seen.  (We tried entering a positive amount for the invoice total and then we could see both lines.)
 
We ended up having to enter the one invoice as two invoices, one for the purchase and one for the credit.  Is this standard operating procedure or is something not working correctly?

So what would be put for the invoice number? because the system wont allow for the same number to be put twice.

OR you can hit the space bar which will leave one space and then enter the same invoice number again.
 
I have tried this method and it does work.