A card payment can only be cancelled up to the point where you confirm the Auth Code. After that you will need to perform a Refund
Cancelling a transaction in this way will generate a Reversal. When a reversal is performed, the customers account will not be debited.
A popup box will appear displaying the Auth Code of the transaction. If, for any reason (eg a signature doesn't match) you click Cancel, the whole transaction will be reversed and the customer's account will not be debited.
The reversal will take a few seconds, similar to the authorisation, and a new Auth Code will be returned in a popup window.