Big bazaar Dharamatala Kolkata - Change my tshirt just size but they are extra charge for that

When I sopping big bazaar and I seeped over the ***** rupees but I buy T-shirt for my friend but size is too long that’s why next day I am going to big bazaar for replacement only size some product but they are charge for me extra. When I talk to store manager he also rudely talk too me and my father. when I buy that Time I used coupon **** and this T-shirt I get *** off ? And this T-shirt price *** I get coupon discount *** when I replace sizes next day they are charge full price ? Whey they are charge that I get discount I just changed my some t shirt only size they are charge me . Why they are give me coupons and they making policy . I am living USA I never seen that policy ? I am very upset. Please do something