You can adjust the refund amount due to a shopper using the Deductions and Fees field.
The Deduction and Fees field is useful when handling manual exchanges, where a customer requests an even product exchange when initiating a return. When handling an exchange, you will want to make sure that you "zero out" the Refund Due to the shopper. This is because you, the merchant, want to keep the value of the original transaction while making sure the customer receives the requested product.
For manual exchanges to occur you will need to create a $0.00 order for the requested product within Shopify, as inventory allows, and issue this order to the customer. Once the item authorized for return have been marked as Delivered, you will use the Deduction and Fees field to lower the Refund Due amount to $0.
Click the Process Refund button to issue a refund (even those for $0) to your shopper's original payment method.
If you are interested in offering automated exchanges with Shopify, please contact firstname.lastname@example.org for more info.