Is a gift given to an employee at a tax center after he has fulfilled the request of the giver considered a bribe, even though he did not ask for it?

It is permissible to give an employee money for his service if it does not entail a Sharia-prohibited consequence, such as prioritizing him over someone more deserving, or causing the neglect of others' needs. However, if his work entails a Sharia-prohibited consequence, or if his superior has not authorized him to take compensation, then it is not permissible for him to take it, nor for you to give it.