Refunds a successful checkout transaction, in full or in part, to the instrument the buyer paid with.

Use it for one-time checkouts and for subscription billing cycles. Get the transaction ID from succeededTransactionId on
the checkout. For a billing cycle, read the cycle's checkoutId from List Subscription Payments, then read that checkout
with Get Checkout, or use Refund Subscription Payment, which finds the transaction for you.

Permission: Merchant Approver.

Rules

• The transaction must be SUCCEEDED and belong to the checkout in the path.
• Only one refund can be in progress per checkout. Wait until the previous refund is COMPLETED or FAILED before you send another.
• The total refunded can never exceed the original charge.
• Some checkouts accept only a refund of the full charge, and some cannot be refunded through the API.
• Send idempotencyKey so you can retry safely: a repeat returns the refund the first request created. Without a key, if the request timed out, call List Checkout Refunds before you retry.

What happens: the refund is created in PENDING and processed asynchronously. It moves to PROCESSING,
then COMPLETED or FAILED (see errorReason). Each change sends the refund-status-changed webhook event
(/reference/refund-status-changed). The checkout gets hasRefunds: true. A refund of a checkout created for a
subscription is typed SUBSCRIPTION, carries the subscriptionId and, for a billing cycle, the subscriptionPaymentId,
and the cycle gets hasRefunds: true. Refunding a cycle does not cancel the subscription.

Errors

• 400 VALIDATION_ERROR: details.amount carries one of INVALID_TRANSACTION_STATUS (the transaction did not succeed),
  CANNOT_REFUND_MORE_THAN_ORIGINAL_AMOUNT (constraints.remainingAmount is what is left, in minor units),
  CANNOT_PARTIALLY_REFUND (send the full amount) or REFUND_NOT_SUPPORTED (this checkout cannot be refunded through the API).
• 404 NOT_FOUND: the checkout or the transaction does not exist, or the transaction belongs to another checkout.
• 409 VALIDATION_ERROR: a refund is already in progress for this checkout; the amount rounds to zero in the currency;
  the total would exceed the charge; this checkout cannot be refunded with this operation; or the idempotencyKey
  was already used with a different amount or transaction.