The ruling on using money in which lawful and unlawful are mixed

Question:
A young Muslim man is afflicted with a father whose unlawful money is mixed with his lawful money—and he is still a student at the university—so what should he do? Should he take from his father’s money what suffices him so that he can complete his studies, or should he leave the university and work to eat from the earnings of his own hand from lawful money?

Answer:
There is no harm in eating from his father’s money as long as he does not know that this thing is forbidden, as long as his father’s money is mixed; so he may take from the mixed money, he has rights upon his father… as long as he does not know that this thing presented to him is forbidden: theft, or betrayal, or usury. If he knows this, then he does not do it. But as long as it is mixed—his money contains lawful and unlawful—there is no blame on him until Allah makes him rich from His bounty.