Purchase contract in advance by fax? Risks as a buyer?

Hello together, a friend of mine wants to see a few used cars in Germany next week and buy one of them. (We are from Vienna). With one of the suppliers (an apparently larger car dealership in NRW (certified world car partner of VW)) it becomes serious now with an Audi, if he enters the price concept of my friend (we assume that the vehicle has no defects and the wear parts are not already borderline), we drive directly to the handle r (and leave the other dealers at the same time), make a test drive and buy the car. The seller now clarifies with his boss whether the price fits and thinks that we should then conclude a purchase contract in advance by fax. Is this procedure ok or normal? Risks for my friend as a buyer? We will of course test the car beforehand and check for apparent defects, and want to, if something does not fit or we have a funny feeling about the matter, no But there is a serious intention to buy (otherwise we would not drive 1000km), then we would drive directly to the dealer, who would actually be the last one on our tour, and leave out the other dealers, where we have suitable offers. I brought my current car also from Germany 2 years ago and there was no purchase agreement in advance. But was also not an official BMW dealer. Thank you already! Nice greetings, T Homas