Taking advantage of customers

I bought the VW door Lock on Friday the 31st July 2020 for an amount of R600. We took the Lock to the repairer and he informed us that we bought a wrong part. We returned the part back on Monday the 3rd and we were informed by Erika that they can't refund me for the returned lock. This is a complete rip-off because this is against the condition of their invoice. I want my refund as in yesterday