Pep cell no refund

I bought a phone at Pep cell Alberton city mall. The box showed 16 gig memory, I took the phone back after 7 days it was very slow, the lady said the phone has a 1 gig memory ram. But she did not tell me when I bought the phone. But on the phone it says 512 mb ram. She said they can't take back the phone because it's not faulty. The phone is an itel phone.