Part not working(bad service)

I bought an oil pump for my golf 6 tsi from alert engine part and when i fitted the part it was not working because the knocking sound did not stop from my engine i went to vw to buy the same part and the knocking sound stopped which indicates clearly that the oil pump from engine alert parts is not working.I then returned the part and they said a technician will call me after assessing the part.Two weeks later i did not receive any respond i then called them and spoke to the manager (Zaheer) and said the part is working but he will give me 20% back which is R250 from the R1250 i bought the part with.I do not understand why i get the 20% if the part is working,.I decided to escalate the matter with the alert engine parts Head office,Lesego said she will help me resolve the problem but it became worse because Zaheer from the joburg branch said i will get the 20% when i buy from them again because i decided to call the Headquarters and i will never buy from them again for the bad service they gave me.

I just want my refund back from the company.

My number is ********** 450