you say you have a signed letter. But you have not told us what the letter says.
Secondly you say "As usual they deny". Does that mean that this has happened before ?
If you have been denied incentive which was promised repeatedly, then why are you still there ?
You can take no action against them (unless of course, this is stated clearly in their letter).
Further, you can go to the labour officer for action under payment of wages act, only if you are having a salary lower than Rs. 18,000. Otherwise, you have to go to civil court, which is nothing but a waste of time and money. Even if you were to go and complain to the labour officer, your existence in the firm is practically over.
So basically if you are not happy, just leave and go somewhere else. There is little other option.
I would suggest you sit with the owner of the firm (or the decision maker) and get clarity on what incentives they are willing to pay you and why you did not get the incentive in the first place.
On the other hand, i know of few cases where HR gets incentives. Recruitment is a part of the job, or even in case it is the main job, i do not know of companies that are willing to pay incentives for recruitment. You do not get incentives for doing your job, the salary is paid for the same. Incentive / Bonus is paid when you achieve something extraordinary.
The only place where such incentives happen is in Recruitment Agencies, as there it is the main reasons for existence of the firm.
8th January 2014 From India, Mumbai