Now my post has been completely deleted. Since it is important for the OP, here is a second AGB-compliant attempt without an external link:
The agreement of the reservation fee could have backfired on the broker. You would have had to pay neither the reservation fee nor the commission (!). Because the broker forfeits the commission due to a breach of the duty of loyalty towards the client if he has a reservation fee or a cancellation fee promised to him. The broker must have acted with intent or with gross negligence approaching intent in a manner seriously contrary to the interests of the client, so that he appears unworthy of his remuneration (BGH, judgment of May 19, 2005, III ZR 322/04).
See also "Forfeiture of broker’s fee due to breach of duty of loyalty towards the client" by the LG Berlin (case no. 5 O 352/99):
1. A broker forfeits his claim to a broker’s fee in corresponding application of § 654 of the Building Code if he causes his client to sign a letter of intent to purchase a specific property, a proposed latest notarization date for the purchase contract, and furthermore an invalid reservation agreement which provides for the immediate payment of a success-independent reservation fee of 1000,- DM.
2. A reservation fee constitutes a success-independent partial commission that cannot be validly agreed upon by means of general terms and conditions (cf. also OLG Hamm, NJW-RR 1998, 1209; OLG Stuttgart, NJW-RR 1996, 822).