COUPON NOT REDEEMING

HELLO SIR, i purchase AOC LED TV on 15 august 2014 and it stop working and made complaint on 16 Sept 2014, they responded on 19/09/2014 and technician came and said display problem and said gone, company people will call and change the display. after 2 to 3 days waiting they dint respond and i called reliance digital and they said they will replace the TV .
in this time i have a coupon which has to be redeem before or on 30th sept. i went on 28 Sept to redeem the coupon they said not now first Ur problem b sort out then u can.
today i went and came to know their is stock of same TV and they dint call me to take that.
i called them today they said come and collect and when i asked about the coupon redemtion they said date is past now u cant use this coupon.
PLs HELP ME .
I want my total money back with interest because they don’t have courtesy to call and they started argument in their shop.