The easiest way to enter an NSF check in Receivables is to enter the transaction as it was originally entered (as a cash batch), with two changes. The dollar amount should be negative, and the check number should have a suffix (12345A instead of 12345).

Then apply the negative amounts to the original invoices, bringing their balance back up to a positive amount. When the unapplied balance is zero, you are all done. Just post the cash batch.

Steve