Poor and rude service

I bought a jacket online on promotion and also used a voucher which was given. The jacket came and it was big and I went in store - Centurion mall to exchange for a smaller size and was rudely served and told that I will have to top up and pay the extra R150 since I had used the voucher on payment yet I was only exchanging for the same item but was told this is because I bought the jacket online. All I wanted was to exchange for a smaller size.