post https://api-test.maast.com/pg/refund/
Returns money to the cardholder from a previously captured payment card transaction. You may request multiple refunds per captured transaction as long as the sum of all refunds does not exceed the original captured transaction amount. Authorizations that have not been captured are not eligible for a refund.
Note: Vendor accounts need special permissions to use this operation; contact Maast Support to enable it.