Used car financing via Santander

Servus together, has someone already had experience with the Santander (car credit)? Have just got an offer about a used car financing available (20.000€ at 4.5% eff.). I find actually quite good. Only drawback: The vehicle letter must be sent within 4 weeks. How does it work with the financing? I have picked out some vehicles, but I can only agree to the dealers if I have the money of Santander in my account. In addition, there is not yet enough says if the car is still available after payment of the loan or if I agree with the dealer at all. By the 4 weeks deadline one would then be forced to find something in time. Has someone already financed about Santander and can say something about it? Can one also postpone the 4 weeks deadline or delay the payment? In any case I want to have the money in my account before I go to the dealer. More problem: The vehicle will be for insurance reasons The Santander has also sent a wipe with the loan offer, which can be filled out if the vehicle is to be registered with someone else. There is also the FIN of the vehicle registered. Can one still submit the slip after the credit authorization / payment? Because before that I do not have any specific FIN.