The blog above deals with debt recovery but its the same sort of thing. If you write a letter of demand requiring the money be returned on return of the vehicle by a given date with the prospect of taking them to small claims court they may just return the money to avoid court.
Given that you have conflicting information on the price I would want to avoid court but it doesn't mean you shouldn't try to recover the money.
That was my idea. I have sent him a text to say I have been given legal advice and I can take him to court. I said I'm happy to settle out of court. I will leave it now and send a letter next! He knows he's in the wrong big time.