You can let the candidate know that you want them to consider you for future employment, but what do you expect them to do for the next 6 months? They're not going to hold off on taking another job. They may consider doing a limited contract term.
If you are sincere and confident you want them in six months, I think you should offer them something.
- Hiring bonus. IF you don't offer them a job in a certain
amount of time, they get to keep it.Obviously if they reject the
offer, they must refund the bonus.
- Referral for contract work. Indicate their qualifications and how unfortunate it was you could not hire
- Pay for Training If you felt the candidate lacked experience in a certain area, offer to send then to training.
Finding top talent is difficult, so I think more companies must go out of their way to attract, hire and retain them. Even if you don't keep this candidate, you're developing a reputation as a company that sincerely cares about employees. You never know, this candidate may know someone else to fill your position.Don't be penny-wise and pound foolish.