Is a gift from a government employee who sometimes takes bribes considered unlawful, and what should be done if that is the case?

If it is known that the gift was acquired through unlawful means (such as a bribe), then it is not permissible to accept it. If this is not known, and the giver's wealth is mixed between lawful and unlawful, then accepting the gift is disliked (makruh). However, if it is taken, there is no harm in that, and it is not obligatory to return it. It is recommended to strive in advising the giver and guiding him towards good.