Supplier refuses refund

On 8 Sept my husband was sent to pick up an engine for our mazda 6 went with the engines block to make sure it's the right engine as previously from other supplier got the wrong engine and had to return engine. Mr Mohammed showed him a row of engines and said all is the right engine choose one. The mechanic said it's wrong but will strip engine to parts and build it into the mazda. Done start and water mix with oil. He called Mr Mohammed to say. Agreed engine will be returened. We without car diffucult to get bakkie. Engine eventually returned as arranged on 13 Nov with telephonic agreement to give money back. Mohammed insist of exchanging engine. The mechanic went to TV2 Motor Spares - Engines himself on 17 Nov also with block to get correct engine. I called the previous day to ask if there will be an extra amount be needed to pay he said no. Then mr. Mohammed said to pay extra money for the correct engine. The mechanic called me and I called Mohammed we agree that he refund me. I gave my banking details he said in two days I will have money. I called today 24 Nov 2015 and mr Mohammed refused refund said mechanic must first come build engine together again.