on world of HR I wrote the following on Bonds/ Contract Letters

This is quite a grey area in the area of training and I don't know
if any companies have managed to institutionalise such a process,
and whether if challenged, it has stood upto scrutiny in a court of
law. Quite a few software companies have such a 'service bond' for
fresh recruits for a period of 2-3 years. You could contact them.

However, I'd wager that any such bond will not be workable in a
course of law.

On the other hand, you'd be sending out a wrong signal to people in
the firm when you make them sign such a bond. It would communicate
to people that the firm does not trust them and is quite
transactional in its dealings. It might even trigger off thoughts of
attrition in people who have had no intention of leaving.

Training however, can be used as a reward mechanism for people. High
end training can be given to people who are high performers and a
clear career path can be made visible to them.

It would be good to keep in mind that causes for attrition are
frequently in the employees immediate work space and trying to clamp
a legal leash on them can be counter productive.