People in the HR business complain about this: a headhunter makes a connection between a candidate and a recruiting officer without asking them both first (the double opt-in introduction), and, when the introduction ends up in a match, sends an invoice for a referral fee.
The same starts happening to me for custom design projects: an introduction, an anticipating client, and the request by the introducer to get paid in a separate email. Two things are wrong here.
One, not going for a double opt-in introduction. Ask both sides first whether they want to be introduced, and if you want to get paid for an introduction, ask me before you introduce, without mentioning the name of the potential client.
Two, I don't work with referral fees. I don't like padding clients invoices with costs that do not benefit from them. And more importantly, and refer lots of other work to other designers when I don't have time to do it myself, without asking anything in return for it.
I felt like I had to make a public statement about this: I am always interested to be connected to new potential clients, but I don't pay referral fees.