Taking a Bonus For Doing A Good Job
Question:

If an employer gives an employee a bonus for doing his job well, is this considered a bribe, particularly if we consider that it is more than the originally agreed upon salary?
Answer:

No, as long as this is an encouragement from the employer, this bonus is not a bribe. However, it is a bribe when the employee will do his agreed upon job only if he receiving a bonus. It is unlawful in this situation because he is receiving extra money for work he is already paid for and is already required to do. There is a clear distinction between giving money as a bonus to encourage an employee to do his job and between giving a bonus to an employee so that he will do his job. An employee must do what he is obliged to do whether he receives a bonus or not.
So taking a bonus is permissible in the case you mentioned. But sometimes an employee may get so used to the idea of a bonus, that he will be expecting it in the future, and when he doesn’t get it, he will be remiss in his work duties. If this result is feared, the employer should not give a bonus because the means to something carries the same judgement at the end.

Shaykh Muhammad bin Saalih al-`Uthaymeen
Fatawa Islamiyah, vol.8, p.100, DARUSSALAM