If you created an invoice in our system and your client then paid you for it with a check, or through some other offline means, you can mark the invoice as being paid offline. Here is how to do that:
From the main menu click on Selling » Invoices.
Locate your client's invoice from the Invoice List.
Click the Options button to the right and select View Installments.
Click the Paid Offline? link.
Set the Paid Date accordingly, and add any Notes you might have for it.
Click Save.
An invoice that has been marked as being paid offline will no longer be payable through your storefront.