Server Handbook: Void a Payment

A void is used to negate an applied payment, removing it from the order. While all voids are tracked in the Action Log, in practical terms a void will nullify a transaction as though it never happened, with no indication of the original payment remaining on the order itself.

While the rules can vary depending on the integrated processing partner, generally speaking credit card voids can only be carried out on same day transactions in an open CC batch; on subsequent days after batch closure, processing gateways will require that a refund be executed rather than a void.  

To process a payment void, the order containing the transaction must first be called up. If the order is still open it may be accessed from the Tab/Table layout or Open Orders. Orders that have been previously closed need to be reopened before a void can be initiated.

From OPEN ORDERS