This is mainly an accounting question. Once you have decided how to handle it accounting-wise, you can execute it in Access. For example, pay the current invoice fully, then take the overpayment and make a partial payment to the next invoice. If the next invoice is not created, go ahead and create one. There are many other ways to handle this.

I would add a field to the Customer table to track Payment Credits. If the amount is greater than zero then the amount can be subtracted from the value of the next invoice.

Do you have provisions for when customers don't pay?  If you have the means to show a balance carried forward, just carry a negative balance forward.  (You may have to make conditions so you don't charge them negative late fees or negative interest, if that is an issue.)  If you don't have the means to carry a balance forward, you will need to create one.

Thank you everybody for your tips

Yeah, this is more of a question about how you want to handle it yourself rather than how to do it in Access. As everyone else has mentioned, there are a million ways to do this kind of stuff. Personally, I have a credit field in the customer record. If someone overpays, I just put a $25 credit on their account or something. If that value isn't 0, it flashes in bright red when I open up the customer's account. But yeah, like other people have said, you can do a negative invoice, or you can do all kinds of different things.