Return confusion

I received as a present 2 pairs of Chinos from someone who bought them at Mr Price on special, the R280 for 2 Chinos special. The date of that purchase was 2020/06/27 from the Eikestad Mall branch in Stellenbosch. I also received the slip so that I could exchange sizes or colours if need be. I went in today to change one of the sizes and was told that I had to pay in for the exchange because the special is not running anymore. I asked how long after a purchase I can return or change items and was told 30 days, but because the special is not running anymore I will have to pay in. I paid in and gave my details, received a new slip, and asked for the old slip back because I wanted to query the transaction as it didn't seem right to me. I got home and saw online that the special was still on and went back to the shop, where there was advertising inside the shop as well showing the special was still running. I explained to someone my situation and they told me its because I only changed one of the pants and not both. I would have gladly changed the colour of the second pair of pants if it meant I didn't have to pay in but I was told that the special is not running anymore. When I presented that the special is still running online, I was told that sometimes it still shows online but its not running instore anymore. I then questioned why the shop is still advertising it instore then as well. I don't mind paying for things that are right, but I am not happy with this situation. Is the special still running and until when? Do I have to pay in if I return only one item from a special like this? Why was I not informed of this when I changed it the first time? Why do I have to hear multiple different reasons why I had to pay in? The special is still online as of 16:31:24 2020/07/01

Can someone please explain?