I've noticed that the Reverse Donation button is not appearing on opportunity records. In the edit layout view, the Visualforce button is on the page layout, but it is not appearing on the actual page. Instead, a Refund button or nothing at all appears in that spot.
If a credit card is processed through CRM, then there will be a Refund button. If the transaction is posted but not processed through CRM, there should be a Reverse Transaction button. The Refund button will refund the transaction that was processed, but the Reverse Transaction button will not.
At times, the Reverse Transaction button may not appear. To fix this, remove the value from the ExternalID field, making a note of the number in case it is later needed.