Convert open balance into credit memo

Hello,

I thought this would be simple, but couldn't find anything in the docs or forums.

We have a customer with an negative open balance, due to overpayment of an invoice.

I would like to convert the open balance into a credit memo (not a refund).

The credit memo form doesn't seem like the logical choice because it is focused on items. There is no item we're refunding. We simply want to acknowledge that the customer overpaid and enable them to use the credit in their next order.

Is there a correct way to do this?

Thanks,

Dave