Be able to only return 1 item to prepaid on multi-item invoice

When an invoice or partial invoice was billed to prepaid it currently requires you to return the whole portion that was applied to PP. She would like to be able to just return 1 item when needed instead of the whole invoice and then billing it again. If the invoice was just applied to a customers AR then you can return just one item, but not when prepaid is involved. If an invoice has prepaid and AR on it, the prepaid items must all be returned but the items on that same invoice that were applied to AR can be returned individually