Cancels a pre-authorisation and releases the whole hold on the customer's card. No money is taken.

• Applies to: a successful pa (pre-authorisation) that hasn't been captured or reversed. Once any part is captured, refund the cp payment instead.
• Whole hold only: you can't reverse part of a hold. Send an empty body.
• 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.