Cell phone repair

I took my phone in last week Monday the 9th of November 2015 and on Wednesday the 11th of November 2015 I received a quotation via sms stating that I've been quoted R603 and if I reject the quotation I'll have to pay a rejection fee of R330. I accepted the quotation so that they can repair the phone because if I rejected they were going to send the phone back with out repairing it and I would of still have to pay the rejection fee. On Friday the 13th of November 2015 I decided to go to the store to find out why are they billing me to repair the phone, the manager couldn't give me an answer then on Tuesday the 18th of November 2015 I decided to call the MTN repair centre to find out why they quoted me to repair the phone they said that the store sent my phone to them without a proof of purchase so they say that the phone is out of warranty and I got the phone last year December 2014 when I upgraded my contract. According to my knowledge when taking a contract phone for repairs the store uses the IMEI number to validate when the phone was taken, now they are refusing to give me back phone until I pay the R603 as the phone was repaired out of warranty.