Angry client

So I went to a Telkom branch and told them I want a laptop. I showed them on the book what I wanted and what to me looked like a laptop. Everything was done and I was told if I am not happy I can return the product within 7days. Mind you I went to the branch on the 25 September 2018 and when I got home after opening the box I was not happy. I clearly explained that I wanted a laptop but what I got was not a laptop it was a tablet. I went back to the branch the following day requesting an exchange to my surprise I was told they can't assist me as the box had been opened. I asked them then why did they say I can return it within 7days if not happy I was told that that only applies to products that are not opened. So now how will one know how a product looks like if they don't open and check it. I am really disappointed and just want to cancel my contract and I am being given the run around.