Mercedes Benz Centurion not refunding money for part

I supp**** Mercedes Benz Centurion with the correct part number and they supp**** me the wrong part which I had to wait 2 days for it to come from Cape Town. I then brought the sample and their system was picking up a part for a A180 CDI as VITO. The parts people had the audacity to tell me that I had changed the engine on my car. I subsequently got the part from another supplier because the first part was R2,800 while the sample based pricing was R5000. The dilemma I am in now is that despite promising to pay back my money in 3 days its now been 7 days and I have not received any news from them.