To refund a payment to your client, you must do it through your payment processor - Stripe, Square, or PayPal. Once you've refunded a payment there, you can mark it as refunded in Sprout Studio.
Go to the Order in Sprout Studio.
On the invoice you'd like to mark as refunded click edit order in the top right.
Add a Custom item to the order for the amount you are going to refund.
To apply the refund, click the "options" button (the three vertical dots at the end of the row) on the invoice you are refunding.
Click "Edit" to partially refund the amount paid or "Delete" to refund the entire payment.