What sales amount is incorrect for you? The MTD statistics or a GL Sales Account. Perhaps the solution is not to use the Sales GL Account when you do the debit note.

You could do an adjustment to the payment, debiting the cash account, but that does not unpay the invoice.

The best solution is to get Adagio Bank Rec which allows you to simply click a button to unpay the customer invoice and also generate the service charges the the bank charges you and what you would charge back to the customer.