cellphone not being repaired under warranty

I recently had an issue with my cellphone not registering my simcard.i took it to Mtn key west for the repair and they wanted the charger also to send the phone for repair.i didn't have my original charger with me and when I asked the consultant if I could buy a charger from them to send the phone in she said its fine and sold me the charger.after a week or so I received a sms from the repair Centre saying my warranty is void because of the generic charger which Mtn sold me.when I went back to Mtn key west the manager said she will sort it out and they took the original charger to resend the phone for the repair.now Mtn key west tells me that I have damaged the simslot and that the warranty will not cover it.i then phoned Mtn in fairland and the repair Centre and they said that Mtn key west is lying to me that there is still an issue regarding the first charger Mtn has sold me.mtn doesn't want to take liability for their mistake and I'm fed up now .my job card no is ***