MerchantTransactionId

We have added merchantTransactionId to the request and response of the POST /refund endpoint. This is your unique
identifier for the API request. We will validate the uniqueness of the merchantTransactionId value so if a
payment/payout/refund is created with merchantTransactionId abc, no other payment/payout/refund can be created with
abc. If this happens we will return a HTTP Status 409 Conflict with a 4701 response code.

This identifier will in future be used to retrieve a payment/payout/refund when you experience a connection drop and
are unable to obtain a paymentId.