Cancels a pending or approved manual approval for a Tier 4 payin transaction,
setting it to REJECTED state.

Use this endpoint when a platform or shopper wants to withdraw a transaction
that is awaiting compliance review. Once cancelled, the transaction cannot
be reinstated — a new POST /pay/rfq must be issued.

The merchant_code query parameter is optional. When omitted, the
authenticated platform code is used as the merchant identifier.