In order to mark a payment as NSF the following must be true:
User has the Mark as NSF privilege
The payment is deposited
The payment method is Check
The payment wasn't previously created as an NSF check
The payment isn't a part of any refunds
Checks that bounce after being deposited can be marked “NSF” from the patient’s Sales history screen. To do this, right-click the payment and select Mark as NSF.
Any fees charged by the bank can also be entered in the Record NSF check dialog.
The original check will be applied to the “NSF check” charge (offsetting it exactly). If it was previously applied to an invoice, it will be unapplied, leaving the invoice with an open balance.