I have figured it out! For anyone paying attention. When you have a paid invoice, it creates a tab on the POS at party start time. The solve here to do what I want is to create a new tab, then move the payment (for both deposit and final invoice) to the new blank tab. This will create a negative balance for the prepaid total. As you ring up items, it deducts from that negative balance to zero. If they go over, they can charge to card on file or you can send an invoice adjustment through catering and events later.
