What is the ruling on taking food products from a brother known for his previous theft—and he might swear falsely—and is it permissible to benefit from them by paying their value as charity to the poor, and is the money mixed with lawful and unlawful elements that his brother gives him considered lawful or unlawful?

If a thief repents, there is no harm in accepting products he gives you. However, if you doubt his repentance, it is more scrupulous not to take anything from him, unless you know for certain that what he offers is the very ill-gotten wealth itself. In that case, it is not permissible to take it; rather, it must be returned to its rightful owner.

As for the expenses he gives you from his lawful salary, there is nothing wrong with accepting them, even if some of his wealth is unlawful.