Unfair Treatment!

On 29 November 2015 I sent a phone to MTN repairs via Pep Store where I purchased the phone as it was still under warranty. When I went back to the store to follow up, they told MTN can't release the phone without proof of purchase from me(which I misplaced).<br> Well, there was no till slip required when they took in the phone and besides the store used IMEI number which validates my phone to be from MTN anyway. I'm being told MTN will charge me R300 to release the phone without a proof of purchase and out of curiosity I asked them to check the value of my phone and it costs R330 at the present. There's just no way I can pay that R300, I may as well just buy a new phone for extra R30 but then is this how mtn wins my phone just because I lost my slip?