I've been working with a client for the last couple of years. A couple big placements with them the last one being 5 months ago.  Since then, I have been working diligently on another couple of roles where one was flushed out quickly that they were just going to out source the need. I spent about 3 weeks on that. They other I've been working pretty hard on the last four to five months and with a change in requirements one time, they final said they are just going to staff it internally. I had numerous interviews on it, the last one being the guy that had everything they wanted. I know there was a couple other firms that they were interviewing from as well.  

Couple questions here for a new contract and invoicing.

Should I implement a clause in my contract to prevent this from happening if the position goes unfilled after 3 months or so if they decided not to hire?

If I was to invoice my client for half or a third of the fee that I would have gotten for filling the role, would that be unprofessional?

Any input would be appreciated.

Thanks, Ed