What is the ruling on the salary of an employee who works for a company that mixes lawful with unlawful, and is it permissible for him to remain in the job if leaving it would lead to harm to his family?

It is not permissible to work for a company that deals in unlawful (haram) things if the work involves assisting them in it. However, if the work is far removed from forbidden acts and there is no assistance in them, then it is permissible, provided that one denounces the evil and advises those responsible for it. One should not fear poverty, for sustenance is in the hand of Allah, and consuming from the unlawful leads to the Fire.