Treating customer unfairly/ worst customer service

I bought iPhone charger at istore Mall Of Africa, is been 7 months now, it just stop working, today I came to their shop telling them that it is not work, they send me to a technician, the technician test it and he confirm that it is not working, he send me back to consultant to tell them that they must pull the invoice for exchange, I was sent to the other consultant, the consultant ask me my details and she said she is creating a job cut for me, immediately I receive sms to say my order is ready for collection, they take me back to another consultant, they took out the new charger, the consultant told me that in 5 minutes they will be done, it is now two hours and am still in the shop, now they are telling me to add more money, they take took decision of me adding the money after two hours while waiting in store, I said but my charge still have warranty , they say they don’t have stock for what I want, they only saw that they don’t have stock after two hours while am waiting. I ask to make immediate payment transfer because I don’t have cards me. They print out banking details I sit with guy down and do transfers in front of him and send proof of payment to his email address, now he they are saying they had to send it to their head office for them to reply, I ask them to send them email and call, they left me again for 30minutes claiming that they are calling head office. Now they came back saying head office does not work on weekend, how can they only find out after two hours that the head office is not working, I pay for the difference that they want now they want head office for what? Two hours in the shop. They are unable to give my charger even when I pay.