Is it permissible to borrow from an employee who works in a usurious bank, and is it permissible to engage him in a business partnership with his money, knowing that it is his salary from the usurious bank?

It is not permissible to use debt as capital for Mudarabah (a form of partnership in which one party provides the capital and the other provides labor). One of the conditions for Mudarabah is that the capital must be cash, readily available, and capable of being disposed of, and debt cannot be disposed of.

Furthermore, Ahmad and most scholars have explicitly stated that it is not permissible to use debt for Mudarabah, because the money in the debtor's possession belongs to him and does not become the property of his creditor until it is received, and in this case, receipt has not occurred.

For the Mudarabah to be valid with your friend's money, the debt must be discharged from the obligation by the creditor receiving it (even through an agent), and the Mudarib (the working partner) should not guarantee the capital except in cases of transgression or negligence. Also, the Mudarib's capital must be known before it is mixed with his friend's money, and each of your shares of the profit must be an undivided share.

If your friend has other money besides his salary from the usurious bank, then it is permissible to deal with him, unless you are certain or it is highly probable that what reached you is from his illicit money itself.