What your brother has signed was almost certainly the contract of sale. The reason you don't pay a deposit in Germany is that generally this is the first and only thing you ever sign. The notary then registers the sale with the relevant public offices and makes sure the seller owns the property/hasn't sold it to anyone else/doesn't owe money on it etc etc. When this is sorted, you pay the price of the property to the seller, who then advises the notary again, who in turn arranges for the transfer of ownership to be completed in the public land registry (Grundbuch).
The trouble with this system for your brother, as I see it, is that once he has signed this contract, he is legallylly the new owner of the property, and if he tries to stop/unpick the process described above without good reason he would be likely to be sued. And not finding tenants within a few months would not be considered a big deal in Germany.
It is a terrible cliche, but the Germans are real sticklers when it comes to their beloved rules and regulations ( I shoiuld know as I am one myself!). I have found, though, that if one just pleasantly refuses to comply - in a kind of "I'm very sorry for your trouble, but no, I won't do that" way - they are often baffled to the degree that they will lay off you. They are not used to dealing with people who are not terrified of rules.... Good luck!