Is it permissible for a government employee in one of the ministries, who receives his monthly salary from the Ministry of Finance, to use this salary for expenses and other needs, given that he does not know whether its source is lawful, unlawful, or mixed?

It does not harm to receive salary through a usurious bank, because the impermissibility of funds is not related to their essence. And the mentioned effect is not authentic. A believer should seek what is lawful without being overly scrupulous in scrutinizing the wealth of those with whom he transacts. Ibn Taymiyyah said: "As for usurped wealth that a Muslim knows to be usurped, it is not permissible to take it from him by any means, because it is the very wealth of the oppressed. But as for wealth whose status is unknown, the unknown is like the nonexistent, and the default assumption for what is in the possession of a Muslim is that it is his property."