A good rule of thumb to follow: if you have to turn someone down for a job either because you're unavailable on their date or don't offer the requested service, always give a recommendation for someone else wherever possible. If you were in the client's shoes, which reply to an inquiry would you rather receive?
I'm not available for your date. Sorry.
I'm not available, but here are a couple of people I trust who might be able to help you.
Whether you call it karma, reaping what you sow, what goes around comes around or just plain being a nice person, being helpful strengthens your brand.