Serious rip-off and bad attitude

We bought clothes at IBO Emalahleni in Highveld Mall ,they didn't fit our son,and decided to exchange them in Boksburg ,the items were on sale at the time ,when we got there the cashier wasn't welcoming and 5 of the 6 items were the same but only changed the sizes to smaller sizes, to my surprise I had to top up by R597 because the shop assistants were saying items went back to their normal price ,how possible is that, we normally buy clothes on special when returning looking for the same items but changing sizes the retailers normally charge the same purchase amount ,so how different is IBO from other retailers,this is a serious *******,and when asking questions people are just impatient and dismissing you.