MTN just let me pay for what I want

I have an MTN phone, one of their own products. The battery is not charging so I went to the MTN store in N1 City Mall to ask what I should do and the lady there sent me to a private cellphone store in the mall who tested the battery and found that it has to be replaced. Unfortunately as it is an MTN phone the battery does not match any of the other manufacturers' products in stock. When I called the MTN service and repair centre I was told that I must book the phone in but if I did not back up all my data then I would lose everything because they have to do a software upgrade. I told the guy that I do not want a software upgrade, just a new battery. He told me that it is standard practice. I kept telling him that I just need a battery and once it's charged, I can back up everything but as it is now I can't back up anything and I don't want to lose my info due to the software upgrade. I do not want the software upgrade, just a battery. Well this went on for a bit and he got very irritated with me. I feel that I should have a choice in this instance and should just be able to get a battery. I am not even allowed to buy one without booking the phone in. So now I am sitting with a phone that means absolutely nothing to me as I only want to access what is on the phone but will lose it if they replace the battery because they insist on doing an upgrade. Totally frustrated!