Sanjeev & Pon have provided valuable inputs. It is the ethics and values of the company because the employee might have worked hard and supported the company in making business. Ethically, he is eligible for the period he worked for; the company might have given it in writing because, in general, companies will provide written documents regarding the variable pay and incentives. If that type of document is available with the employee, then he may act accordingly.
In addition to the above, by not paying the incentive to the employee, you are raising suspicion/doubt in the minds of other employees who are still with the company, and there is every possibility of a reduction in the productivity of those people. The company's image/reputation is also an issue.
Regards,
Kamesh