Faulty Spare Parts

i purchased a Power Grip Timing Belt TOY163 that on face value is the correct part number,when fitted into vehicle is loose,when i have measured it up to the old belt its about 6mm out of size, he refuses to get me another brand or my money back and wants me to change the tensioner,what difference will replacing a fixed radius do, when the belt is wrong.While i was there spoke to his supplier,also said im wrong,also his partner asking me if im a Mechanic,i dont care what they say, if I am right I am right,taken measurments and pictures taking this up with POWER GRIP,GATES. USA also.