What is the ruling on dealing with someone whose wealth is mostly unlawful, and does this disapproval entail a doubt concerning the money acquired from him? And what is the ruling on dealing with someone whose wealth is mostly lawful but contains a small portion of unlawfulness?

It is reprehensible (makruh) to deal with someone whose majority of wealth is unlawful (haram), unless there is a necessity. The proof for this is the Prophet Muhammad's (peace be upon him) dealings with the Jews.

It is not forbidden to eat from wealth resulting from transactions with someone whose majority of wealth is unlawful, nor is it forbidden to invest it or develop it. It does not prevent the answering of prayers or harden the heart, unless it originates from an inherently forbidden transaction, such as stolen or usurped property.