How to create/process a refund for a customer after voiding the receipt

A customer's credit card was charged for monthly rent by accident, and the receipt was voided so the payment no longer shows, but when I clicked "yes" to refund the payment amount to the customer, a notification popped up that said the refund was unable to be processed. So now I'm not sure what I need to do to get a refund processed for the customer, any advice???