I'm not happy

Good morning I'm so upset about my experience at Mr Price Kempton Park. I bought a shirt and a track pan together on a sale of buy one and get one free unfortunately the pants are small and they not the right size so I tried to return them and they told me that they can't return it because I got it for free and they made a joke out of it that I should give these pants away. Have I been informed before I bought that they don't exchange a free item, I would have known that I should fit it 1st before leaving the store. It came as a shock that I can't exchange it to something else as they don't have the bigger size on the pants that I bought. As a Mr price customer I'm not happy and I hope something gets done. All I'm saying is that I was supposed to be told before buying....