When making an introduction do you ask for permission?

When someone asks to meet one of your friends, do you fire off an e-mail right away with both parties on it or do you contact your friend and ask for permission to introduce you to a person?  It kind of depends on the circumstances.  Where things get most sensitive is one party could be a buyer and the other is a seller.  I would say you definitely want to get permission from both parties to connect before you make an introduction.