Misleading

Extremely disappointed and dissatisfied with a exchange policy you guys go against

I bought a pair of shoes that was priced at R775 and used a R200 discount voucher bringing the price to R575, however the shoe was big and upon arrival the store was closed and asked to come back the next day been a Monday to exchange for the correct size but the people at the store made me pay the full price and removed the discount saying that it will only apply to the shoe been the wrong size the only difference is a size smaller why should I lose my discount just to get the correct size