Yes, you can issue a refund for a transaction even if the customer no longer has billing information stored on their account. The refunded amount will always go back to the original card that was used to pay for the transaction. You can do this by visiting the invoice details page for the charge invoice, go under Invoice Actions in the top right-hand corner of the page and then click on Issue Refund. If the invoice also has a credit adjustment, you'll want to ensure that when you're viewing the refund preview that the refund method is set to Original Payment Method as it will not be sent back to the customer if it's set to Refund Credit First. You can also find the same refund page from within the transaction's details page, simply visit the paid transaction and click on Refund Transaction.