Is it permissible to accept a gift whose source is unknown, especially if it is from a husband who works in the police force, and is it considered a bribe?

What the police chief takes from people in exchange for his obligatory work is forbidden bribery, and it is not permissible for farmers to pay it unless it is a means to repel injustice from them.

If the police chief's wife gives a gift, and the questioner does not know its source, she may accept it. However, if she knows that it is from illicit money (bribe), then it is not permissible for her to accept it.