I understand from your post that everyone is getting about one lakh except you for incentive right. But as far as incentive is concerned it is given for the employee who performs well in their job and completes their target successfully.
So why is that you have returned your incentive back based upon your performance at work they have given you the incentive. And one more thing is what you have written to your manager is absolutely wrong if you have any concern about your less incentive you should have spoken to him in personal rather than returning the amount with that write up of yours
17th November 2009 From India, Madras