Car purchase and registration

Hello together, I picked out a car at mobile.de and would like to drive to the dealer next Saturday (he doesn’t know anything about his luck yet). The problem is that it’s 2.5 sts drive and 2.5 sts drive back as well. As I, provided the car is as described in the advertisement, only want to drive 1x now I have a problem with the transfer of the car. Because even with a short-term registration mark I need the vehicle letter or appearance which I do not want to drive yet on Saturdays the registration office also has to. On the internet it is that vehicle is immediately available and therefore I assume that you can take it directly with you. That’s certainly not a new problem but I don’t find any foren entries to it. Can you tell me how best to solve this when I want to buy the car on Saturday and take it directly? Thank you Markus