Q. Then what about that amount paid RCM igst which was un utilised credit . because of import / export zero rated ?
Ans. Under RCM tax is paid in cash and after making payment of IGST in cash, you can take credit of IGST paid under RCM. In case your credit of IGST remains unutilsed because of export (zero rated supply), you are entitled to refund claim on this ground. Your original query talks of refund of IGST paid under RCM and it is entirely different issue.
You are entitled to refund claim of unutilised ITC. Pl. note that it is not refund of IGST but refund of credit remained un-utlised because of export.
This is not refund of IGST paid against import of service under RCM. See the difference between both situations.