Exchanging of parts

I bought piston on Monday I was assisted by Kat Indian guy in Pretoria west and I want to return them because he gave me wrong things n he said I can’t return cause I tried to fit them.so I ask obviously when I buy something from nhen I’m trying to fit they don’t fit now I’m stucked with parts I can’t use r2800 it’s a lot of money just to give alert part for no reason especially my customer needs were not met