Authorize.Net Virtual Point of Sale : Using VPOS : Reversing a Charge
 
Reversing a Charge
Step 1 On the New Transaction screen, click Credit Card Refund or Void.
Step 2 Enter the transaction ID of the charge to reverse.
■If the transaction is unsettled, it will now be marked as Void. Void transactions will never settle, but will remain in your Transaction History.
■If the transaction is settled, we will create and automatically settle a new Credit transaction, linked to the refunded transaction.
You can also manually void unsettled transactions and manually refund charges in the Merchant Interface.
You also have the option to void a transaction immediately upon charging the card. Click Void Transaction on the confirmation page.