You may want to follow along in MyCase as you follow these steps:
To refund a payment, open the Invoice Details page for the invoice to which the payment was applied.
Open up the invoice you would like to refund.
All payments applied to this invoice will be listed in the Activity section at the top of the invoice.
Find the payment that you would like to refund.
On the row for that payment, to the right of the screen, click the Refund button.
You will be given the option to refund the full payment or a partial amount.
MyCase also has a payment Delete feature. Unlike the refund feature you just learned about, deleting a payment completely removes the payment (and leaves no paper trail). This is useful for erroneous or double payments that have been applied to an invoice. Learn more: How do I delete a payment made on an invoice?