How do I proceed best now?
1.) Secure evidence (screenshot, printout of the listing showing the brokerage fee of 3.57%). 2.) Under these circumstances, I would have the draft purchase agreement reviewed by an independent lawyer you commission and pay for (in Magdeburg I have an address, which you can find yourself with Google's help, otherwise PM). 3.) Reject the broker's higher brokerage fee claim (brief letter). 4.) Pay what you owe (3.57% of the purchase price, but of course not before the brokerage fee is due!!)
Reservation fee ... Hmm, just don't pay it, dispute the claim (justification BGH etc.) ... If it is finally offset, maybe that's okay too. Learning cost ...
Soil survey? No idea ... You want it, you will certainly have to pay for it. The question remains whether that could be done without the broker, but the order has already been placed ...
Building ... Definitely not with this broker and/or partners. Hopefully, you will buy the property free from any developer (see also 2nd review of the draft purchase agreement).