Question
What are the rules about payment of a fee to a shadchan (matchmaker)?
Answer
There’s an excellent summary of all the laws related to matchmaking, including payment of fees, in the book Ha-Nisuin Ke-Hilchoson Ch.4; the footnotes there give many references. On the fees see, in particular, Even Ha-Ezer 50, and especially Aruch Ha-Shulchan 50:42 and Pischei Teshuvah 50:16.