Cancels a pre-authorisation and releases the whole hold on the customer's card. No money is taken. A reversal is always for the full amount.

• Applies to: a successful pa (pre-authorisation) with nothing captured and not yet reversed. BillPro refuses a pa that's fully captured or already reversed.
• After a capture: you can't reverse. To give money back, refund the cp payment instead.
• Full amount only: send an empty body. A partial reversal isn't possible: if you send an amount, the gateway declines it.
• Declined: if the gateway declines the reversal, it's still saved as a new rv payment, with successful: false.
• Afterwards: the reversal is a new rv payment. Its original_reference_id is the pre-authorisation's original_id. The pa's refunded becomes true, although no money was taken. There's no separate reversed flag.
• A reversed pre-authorisation can't be captured.

A reversal can't be undone.