GST ON DISCOUNTED MERCHANDISE

I HAD PURCHASED SOME MERCHANDISE FROM THE BRAND FACTORY OUTLET AT AGRA TODAY. THEY WERE CHARGING ADDITIONAL 5% GST ON DISCOUNTED PRICE. TO MY KNOWLEDGE, GST HAS BEEN ALREADY APPLIED ON THE MRP AND THERE IS NO POINT IN APPLYING ADDITIONAL 5PC TAX ON DISCOUNTED AMOUNT. IF AT ALL THEY HAVE TO ADD GST, THEN THEY SHOULD REDUCE THE TAX AMOUNT FROM MRP, THEN GIVE OFFER AND THEN APPLY GST ON THE REDUCED PRICE. OTHERWISE THEY ARE CHEATING GENERAL PUBLIC ON THIS. WHEN QUERIED THE GUY AT THE BILLING COUNTER GOT ANNOYED AND WAS BEHAVING RUDELY, SAYING NOT TO WASTE HIS TIME AND NOT TO TAKE THE ITEMS IF I CANNOT PAY THE TAX.