Trying to Exchange items for the same but smaller size

I bought 2 pants last week Monday 28 May 2018, as my 2 year old was with me and starting to get difficult, I did not have time to fit them. So I just bought them and fitted them at home. They were too big and I only got time yesterday 4 June 2018 to go and exchange them for a smaller size.

When I bought them on 28 May they were on promotion for, buy 2 for R150-00 and the normal price is R99-99 each, When I tried to exchange them yesterday for the correct size the store (Reds mall, Centurion) refused unless I pay in the R50-00. Their reason for this was that it is not on promotion anymore.

Surely that cannot be correct, I am not returning them for something else or for a refund, I wanted the exact same pants I already bought and paid for, just wanted to exchange them for a smaller size...... I also wanted to buy 2 more which I told them I will obviously by the full price for, but I am not going to pay in R50-00 for the 2 already bought last week.

They did not want to listen to my reason and I ended up telling them, to give me back my slips and pants I bought, but now I am still sitting with the 2 pants that is too big and I really wanted to get 2 more as it is really beautiful pants, What do I do now? What if it was reversed, what if it was not on promotion when I bought them but was wen I wanted to exchange, surely they would not have given me R50-00 change?

The staff and store manager was very unhelpful and did not even care to listen to me.