Woolworths refused to do a swap on a in-corrected sized item without me paying in more money

So on the 19/12/2019 I went to Woolworths Canal walk and purchased 2 swimsuits, getting 30% which was a good deal but when I got home and noticed that the 1 swimsuit hanger size was obviously was wrong because the swimsuit was 2 sizes too small as I just took it according to the hanger size i decided i needed to return it. So on the 20/12/2019 I went to Kenilworth woolworths as chose another swimsuit as the one i was returning they did not have stock of. So I still chose a swimsuit which was R50 cheaper than the original one I was returning. I explained to Naeemah Lindewal (Cashier) of the above and she said they cant do an exchange because they can only refund the discount price of the item R315 (original price -R450) So because the one I wanted was R399 , I would have to pay the difference, I had a manager called but was told the same thing. So because no one wanted to help me and I needed the swimsuit urgently I had to pay a extra R84. Which i think is robbing me ! because the special is to buy 2 and get 30% off which is what we did, I just wanted a swap for a bigger size yet they got a extra R84 +R50 out of me because the one returning was more than the second one. This being woolworths with certain standards I am so angry because this is robbing me and nothing is cheap in that stores !!!!!. I want this seen too , I have all the slips.