Hi talinga,
I understand that you need help with how the customer credits work in the application. For your first question, the overpayment should produce a credit for that customer which can be used on other invoices or refunded to the customer. You can access this from the sales > sales returns and credits.
As for the second issue, it might be best to recreate the refund from the sales menu first and match it with the bank feeds instead of creating the return from bank feeds directly. Here is a guide for that.
Let us know if you need any further help.
Thanks,
Genreve
