You would generally be doing this if you do not expect to get paid for some reason. Your advisor will give you the best direction as to when and how to write-off Sales to a customer.

1. Load the original Sale you wish to write off
2. Click the Duplicate button to make a copy of this transaction.
3. When the new Sale is loaded change the sign on the amount for each line item to be the opposite sign for each line item. e.g. A positive amount would be changed to negative.
4. Enter the transaction date to be that which you intend the write off to be accounted for
5. Click on Save.
6. Use a clearing Bank Account (or create one called Asset: Clearing). Apply a payment of $0 as at the date you intended the write off to be accounted for to both the original Sale and the new Sale write-off. In order to do this go to Receive Payment screen by clicking on the Add icon next to Payments under Sales. Enter $0 for Total Amount, enter the full amount for the original sale and the negative full amount for the new one you created. You are creating a $0 payment, so there is no cash flow effect on the account.