Zappos is a great example of doing this kind of thing. From anecdotes, it sounds like if you call Zappos to order something that they don't have in stock, they'll help you find a competitor that has it. There's some insurance companies that do this too. They'll help get you quotes from competitors. Amazon does this a bit too with their outbound advertising links. (Of course for Amazon there's still a direct monetary incentive)

Sometimes it's a great hook to help potential customers feel more comfortable doing business by giving them a good reason to trust you. You're doing what's right for them as a customer, not necessarily what's right for you as a business.