Cna won't take my brand new phone back for warrantee repair

I went to CNA Pretoria church street. I bought a phone, but to my surprise they gave me the phone that was on display . I was in a hurry so I accepted the already opened phone that was on display because I had already paid. I got home and the battery was already flat. The battery doesn't last at all even after taking three hours to recharge. I took the phone back but they refused to take it back saying I had somehow ****ed the battery myself. The phone is new and I have never used it before because of the battery issue. I bought another battery because I had no choice. But still. The new battery doesn't last. The fault is on the phone itself. Now I'm stuck with a faulty new phone and a new battery. Cna won't take it back for repairs.