The answer is you cannot.
There was a confirmation screen and you were asked twice, if you wanted to issue a refund. Once a refund is scheduled, it will go through. A refund can take up to 10 days to show up on your client's account, as it potentially traverses several different banking institutions, which is out of our control.
The easiest way to remedy an erroneously issued refund, is to process a new transaction with your client.