Unhappy customer Mall of the South

bought a dress online last week Sunday size 30,the size was a little bit smaller. Today I went to Truworths in Mulberton for exchange.I find the size 32 that I wanted,I went to ther till with both items for exchange.They did the transaction,at the end of it they told me that I must pay a difference of 10% because the dress had a 10% percent discount on purchase. I asked how is that possible because its just an exchange. I asked them to give me back the dress that I came with (30) because I ddnt have money to top-up,and I have already purchased another dress . They told me that I cannot get the dress back the only thing they can do is to refund me if I dont have 10% which was R53. Or refund the dress I just bought to top-up the exchange. I asked why they ddnt tell me before they do the transaction that I will have to top up?They said that was supposed to be explained on purchase,because I purchased on line then ther was nobody to explain that. While was digesting that,they started to do a refund. I am very angry and disappointed at the same time.On top of it all,they had a very bad attitude.