Exchanged size but had to pay for free item

Good day

I bought 3 fitted sheets and got one free. On the 27th of December 2017 I went to exchange 2 fitted which I took the wrong size. When I got to the store I explained to the lady. She then informed me that as per her system it shows that I have to pay R159.99. I explained I don’t want a refund im just wanting to change the size. I requested for a manager she then told me I have to pay an additional R10 as per the system. I them explained myself to her as well. She too insisted as per system. The customer service was extremely poor as I too work with customers. I informed then asked the manager so then the “free sheet” as per the buy 3 and get one free is then not really free now as I had to pay an additional R10 to just change the size. My question how many client do they rob like that. As I was first told to pay R159.99 when I refused I was told to pay R10.00. What is the purpose of saying 3 and get one free. They should then add if you take the wrong size upon wanting to change the size you will pay extra