Pep Cellphone Return Policy

I bought a phone today 8th of March 2021 with the intention it was a dual SIM. After I paid I realised that the phone wasn't a dual SIM phone,I immediately alerted the cashier that I wanted a dual SIM Nokia 105 and that this one was Nokia 105 with no dual SIM card slots.. she refused to return the phone even though I had not left the store with it. She insisted that the box was already open and noone would buy it once it was open..I haven't used the phone as of yet. I would like some assistance