IGST ON COMMISSION INCOME - Goods and Services Tax - GST

Dear Expert We are Importer and Trading Goods in the Local market, We have to pay Less Amount to the Our Foreign Supplier, We have Accounted Less Amount Under the head Commission Income under balance Sheet, Now Under Department Audit, GST Officer has issued show cause for Leavy IGST Amount on Commission Amount Under Section 13 of IGST Act , ( Section 13(8)(B) , They are Treating us Intermediary for such Commission . How to tackle , Please Guide