The Association of Fundraising Professionals says it is unethical for a charity to pay a finder’s fee to a person who brings a donor to the organization, but it is not illegal.

Depending on how the finder actually operates, the finder may need to be registered as a professional fundraising counsel or solicitor if operating in one of the 39 states and the District of Columbia that have charitable solicitation registration laws.  A charity operating in those jurisdictions is generally not permitted to deal with an unregistered counsel or solicitor and has to list them on its registration statement.  If you decide to play this game, both of you ought to check any applicable registration statutes.  It can significantly interfere with your operations if a state says you aren’t in compliance with its registration law.