Any Credit Card or ACH Donation processed with the gateway can be refunded in Stellar. To do so, you will need to access the Transaction Details page for the donation. There are two ways to do this.
The first is to go to the Donor's Contact Page and select Financials from the left navigation. Select Donations and then click on any Donation to pull up the Transaction Page.
When on the Transaction Page, scroll to the bottom and click the Blue Refund link.
A second way to access the transaction details is from the Pool Report Dashboard Module.