To change a product to provide a credit for an early return, first go to Products>Products List. Click the View button for the product you wish to edit. On the product display, click the Pricing tab. You can set the Credit On Early Return to No, Yes, or Prompt Each Time. The latter option will prompt the user (employee) if the item is returned early to determine if a credit should be applied.
After editing the product, make sure you click the Save button.