assuming you are paying cash, when you exit Germany with more than €10k cash you have to declare it to customs. I did it for peace of mind as I was flying over and the risk of getting asked silly questions at the airport, especially in my case to Switzerland, is small but real, but unlikely if you drive there. If you declare it on exit, be ready to send over a copy of the sales contract to German customs, in my case they did ask for it a couple of months later... If for whatever reason the deal falls through and you can't provide a copy of a sales contract, they will probably ask you to provide evidence that you put the money back in the bank in Germany. There is a form to fill in, some infos here:
Make sure you are precise in counting all the money you are taking with you and that it matches what you declare in the form, even including coins.