Automation help so that only one specific record is changed

I have a relationship between two tables, "Customers" and "Invoices". In one of the shared fields from Customers, I have a multi-choice field "Rebate Used from Last Month?" of a simple yes and no. I currently have the automation that when a new invoice is saved, it will automatically change the answer from No to Yes. That's great. The problem is that that new saved invoice will no longer apply the rebate. Which I understand why it is functioning that way. What I need is for the rebate to be applied to that first invoice and still be applied after the question is changed to Yes. Any help would be greatly.