If your customer has a credit balance on their account and you're not sure why, I recommend checking the payments linked to that customer's latest invoices and make sure there isn't a duplicate somewhere. If you find that there's a duplicate payment linked to one of the invoices, you can follow the steps below to delete it.
Go to the Sales menu.
Find and open the payment.
Review the payment. Make sure you need to void or delete it.
If you want to include the void or delete date, add a note to the Memo or Message section.