Sold an old spare part that is not working and refusing to pay refund

This company is taking advantage of clients and taking them for granted. My brother in law bought the fuel injector for Jetta 6 on Monday 21 Feb 2022 from Boss Auto Spares. The part was not in their stock, they ordered the part and they promised him that he will come to collect it in the afternoon because it was during the morning. When he went to collect in the afternoon, they postponed collection date till Wednesday and then postponed till Friday. On Friday 25 Feb 2022, he sent a driver colleague to collect the part on his behalf. After he received the part he realized the part he was given was used. He took the part back for a refund today, they took that part but they are refusing to refund after agreeing to pay him after 24hrs. The part cost R1350.00 and they initially agreed to pay. Now they say it is not the part they gave him. We suspect that the many days they kept postponing the collection was because they were still sourcing the faulty parts to send him. This is appalling behaviour and they should not get away with it.