Unfairness

I've bought Adidas Adimule on Bash online they were on promo from 1900 to 1300 with some cents now I want to change for bigger size they said I must exchange on a normal price of which it unfair because I've always wanted this Adimule but because they were expensive I couldn't buy them now that I've finally got them on promo n unfortunately they are smaller cut I wanted bigger size I want exchange on a normally price that they told me In benoni lakeside mall and the was nothing like that online purchase