Is it permissible to participate in a business project with someone whose wealth has been mixed with what is forbidden?

Question:
He says: Honorable Shaykh, may Allah grant you success. One of my colleagues offered to let me partner with him in a business project, but his wealth is forbidden. Is it permissible for me to partner with him? Because one of the students of knowledge permitted this partnership on the claim that the Prophet (peace and blessings be upon him) used to trade with the Jews, and their wealth was forbidden.

Answer:
This is a lie. Not all the wealth of the Jews was forbidden; rather, some of it was forbidden and some of it was lawful. If a person’s wealth is mixed with lawful and forbidden, there is no harm in partnering with him or eating his food, on the basis that it is from the lawful; because you do not know which part is from the forbidden, and the default is permissibility. The Prophet (peace and blessings be upon him) used to eat from the food of the Jews and buy from them, because not everything they had was forbidden; they had a lot of lawful and they had forbidden. However, if you know that this person’s wealth is entirely forbidden, it is not permissible for you to eat from it, nor is it permissible for you to accept a gift from him, nor to buy anything from him, because it is forbidden.