The payment to refund.

Amount to refund, as a decimal string. Omit to refund everything still refundable on the payment. Must not exceed the amount captured less any refund already issued.

Optional reference of your own for reconciling the refund.

Optional caller-supplied key that makes the call idempotent. Repeating a call with the same key and org is answered as the first call was — with the refund it created, or with its rejection — and never refunds again. A repeat that names a different payment or amount is rejected. Strongly recommended: without it a retried request issues a second refund whenever the amount still fits the balance.