How do I rid myself of doubt concerning people's money that may contain unlawful transactions, and how can I borrow lawful money without fearing it will become unlawful again?

The repentant individual who previously engaged in illicit financial transactions is congratulated, and we ask Allah to grant them lawful provision. The fundamental principle regarding the wealth of Muslims is that it is acquired through legitimate means, and it is permissible to deal with it unless proven otherwise, as Shaikh al-Islam Ibn Taymiyyah affirmed. Unlawful wealth is of two types: The first is that which is unlawful due to its acquisition, such as interest-based wealth. This is forbidden only to the one who acquired it and is not forbidden to someone who receives it through a legitimate transaction. The second is that which is inherently unlawful, such as alcohol or stolen money. This is impermissible to accept. The general principle is that wealth unlawful due to its acquisition is not forbidden to anyone other than the one who acquired it, unlike that which is inherently unlawful.