Unfair treatment

I purchased Oil Pump from All Merc Parts in Pretoria,when I went there I was told to busy Sub Assembly by the mechanic.The sales man advised me to buy Oil pump instead it works out cheaper that way,I bought it because obviously I trusted him as I believed he will not mislead me. I then took it to the mechanic who told me I bought a wrong part,I went back the shop to return the part and Ally the sales guy told me they cannot take it back because the part was correctly supp****, I then went back to the mechanic explained to him and he asked me to asked them then to atleast swap it for something else,they told me that the only way to take it back it is when I buy a full engine which is R35 000 more than the part I bought I have been trying to resolved it but they all sing the same song, meaning I am stuck with a part I don't need meaning my car can't move,which means no business for me as I use it as a form of transport.The service I received was very nice before I bought but after that I experienced rudeness. At this stage all I want is my money,I have been asking to speak to the manager since today, they just don't want the manager to speak to me. I am 540km away from their shop to come back and forth with this. I believed this is an Unfair treatment to people,no TCF from their business yet they deal with customers.I have their calls recorded if they are needed

Merc All Parts contact: ***

*** Vinny