What should a Muslim do if he learns that the gift, food, or money he borrowed originated from an unlawful source, whether he knew this before or after accepting it?

The basic principle is that what a person possesses is their property, and it is permissible to deal with a trustworthy Muslim without scrutinizing the source of their wealth. Unlawful wealth is of two types: 1. That which was taken unjustly (e.g., usurped or stolen): It must be returned to its owner if known; otherwise, it should be given as charity on their behalf. If it was a loan and it is possible to return it to its owners, that is obligatory; otherwise, it is returned to the lender. 2. That which was taken with the owner's consent (e.g., usurious interest): If it is known to be the very essence of unlawful money, it must be avoided. If unlawful money is mixed with lawful money, there is no harm in dealing, accepting gifts, or borrowing, and there is no need for compensation after its destruction.

There is no obligation to investigate the source of wealth; rather, it is a blameworthy over-scrupulousness. Sheikh al-Islam Ibn Taymiyyah said: "Whatever is known to be unlawful should be avoided, and what is not known is like what does not exist, and the default is that wealth is lawful." There is no doubt in dealing with a trustworthy Muslim, and whoever refrains from dealing with them out of scrupulousness has innovated.