Faulty petrol pump

I've purchased a petrol pump today 12:47 from Diesel Electronic Outeniqua ***, I've took the pump to the Mechanic to find out the pump is not working 15:20 the mechanic gave me a call the pump isn't working because he took another car one out and it's working, so I Winston a call 15:29 to tell him, he said they don't exchange it, but he never told me that and it's not written anywhere on the slip. I really feel this is unfair because I don't want my money back I want to have it exchanged.