Authorize.net makes it easy to void or refund a transaction, but you need to know when to void a transaction verse refunding a transaction.

Void a Transaction if the transaction was made earlier on the same day and the transaction has not yet been batched out at the close of the business day.
Refund a transaction if the transaction was made the previous day or many days earlier. This transaction would have already been batched out, so it is not eligible to be voided anymore.

Read the instructions below to learn how to Void a transaction. Go here to learn how to Refund a transaction.

After logging into your Authorize.net account, click on the link for Unsettled Transactions.
These are all of your transactions from the current day which have not been batched out yet for settlement.

When you see the transaction, click on the Transaction ID for more details.

At the top of the Transaction Details screen, click on the Void button.

Click OK to confirm that you want to void the transaction.

Now, when you view the transaction, you will see that the status is changed to Voided.

When a transaction is voided, the funds will show up in the customers account within a day or so, depending on how their bank handles the funds authorization and release. Once a transaction is voided, you may need to record the cancelled payment in your POS, shopping cart or accounting software if applicable.