I was having difficulty doing a shidduch with my son. I knew that my own shadchan was not happy with the way my mother in law treated her. I went to my shadchan’s house and gave her an envelope saying that my mother in law decided to do the right thing. Well, six weeks later my son got engaged. At that point some of his classmates already had several children. I didn’t say anything to my mother in law, but was worried that the shadchan might thank her when she sees her at the wedding.
I don’t know if his getting engaged had anything to do with the shadchanus being paid, but who cares.
As far as I know a shadchan has to be paid.