Problem
You need to refund a payment that has already been exported, but the option to do so is missing.
Solution
You will need to create an adjustment invoice to generate a balance on the invoice, apply the payment, and do a manual refund on that invoice.
Refund an exported payment when the option to refund is not available
Search for and go to the customer's profile.
Find an existing invoice for the same customer.
Perform the steps outlined in Create an adjustment invoice to apply credit or collect payment.
Process the refund for the now applied credit.
Apply a negative payment to the invoice to bring the balance back to zero.