If you know that a customer is not going to pay you for an invoice, you should write this sale off, typically as bad debt.

To do this:

1. Create a new account named Bad Debt and assign it an expense account classification (6-xxxx).
2. From the Sales module use the Enter Sales window to create a Credit Memo.
3. After selecting the customer on the credit memo, click the Layout button at the bottom and change it to a Service layout.
4. For each of the customer's outstanding debts, enter a line item and assign it to the Bad Debts account you created in step 1. 
   Note: If you use Categories, make sure you enter the same category on the credit as you did on the original sale.
5. When you're finished, click Record. The transaction you've recorded is called a customer credit.
6. In the Sales command center open the Sales Register, then select the Returns & Credits tab from the top.
7. Highlight the customer credit you created in step 2.
8. Click ‘Apply to Sale’, opening the Settle Returns & Credits window.
9. Apply the credit memo to one or more of the customer's open sales, which appear in a scrolling window.
10. Click Record.