PO on order after return to supplier

We're on 11.5.9. Here's the story:

PO quantity is 6 and those 6 are received and delivered. And the invoice is matched for 6. Status was closed for receiving, then closed for invoice and then simply "Closed"

Months later an out of box failure occured for 1 of the 6 and the supplier agrees to the return. We do a return to supplier transaction for 1. That reopens the line for receiving but the line stays as closed for invoice. I assume the line reopens to accept a replacement at no charge.

But the supplier can't provide a replacement and we want to change the Po quantity from 6 to 5 but can't because of the closed for invoicing status. It may be weeks before the credit can be applied.