Pick up used cars only a few days after payment?

Hello guys, I looked at a car this week which I like very much, I have already signed a purchase agreement with the dealer. Paying I do in bar but only next week. The problem is, I get the paper etc. only after I have given the money to the dealer in bar. That means the car has to stand still at the dealer until I have it logged in and have my number plate. Since the dealer lives a little further away it has to go so unfortunately, red number plates want I don’t get the price for the fuel to go there is cheaper than to make the circus with the number plates now. Now to my real question. If I have the purchase contract, car papers etc and the car and pick up later, but the dealer suddenly says “never sold” or anything else turns crooked, he can then somehow win in court or…? just worry me a little, because the car is still a few days after the payment with him until I have just registered it and have the number plates. Greetings, Elephantoz