All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The ruling on making a partnership with a person whose money is mixed with unlawful money is as follow:
If the person intends to be a partner with someone and was able to determine the very source of unlawful money that the latter has, then he isn’t permitted to make this partnership. Yet, if the latter’s money was a mixture of lawful and unlawful money such as paper money, which makes it hard to verify their source, then making a partnership with him is permitted yet remains reprehensible.
Furthermore, the scholar Ibn Hajer Al-Haythami (May Allah have mercy on his soul) was asked in this regard:’’ It is obvious that Jews and Christian sell wine and make a lot of usurious transactions and the like. Is exchanging gifts and dealing with them permitted? And is dealing with the one whose most of his money is unlawful prohibited? He replies: if you (questioner) haven’t seen them making unlawful transactions then it is permitted to initiate transactions with them and to accept their gifts, since the Prophet (PBUH) accepted the gifts given from Christians the Jews. However, if you were able to verify the source of their money .i.e. witnessing them selling wines and take money in return, then you aren’t permitted to dept from or deal with them according to the teachings of Islam. And the same applies on dealing with money that belongs to unjust people or with a person whose money are mostly unlawful. Therefore, it is reprehensible to make a deal or a transaction with a person without being able to verify his source of his money.” [Al-Fatawa Al-Fiqhiya Al-Kobra vol.2 pp.233].
Finally, it is stated:” Making a partnership with a person whose money is a mixture of lawful and unlawful money is valid although reprehensible such as making a partnership with a Dhimmi (A non-Muslim living under the protection of a Muslim state), with a person who makes usurious transactions or with a person whose most of his money are unlawful.” [Hashyat Al-Qulobi A’la Shrh Al-Minhaj vol.2 pp.418]. And Allah knows best.