Unacceptable service

I went to the store on Saturday and took the sample of the clutch I needed for my vehicle (removed from my vehicle) to ensure that they do not make a mistake. When I got home the clutch did not fit and they indeed sold the incorrect item. When I went to the store this morning they refused to accept the returned item (including the slip as proof of purchase). Their response was that their staff is only human and they do make mistakes. They are refusing to refund me or give me the correct item I requested. I am recently unemployed and spent my last money on this item and I do not have money to just buy another item because of their error.