Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh
If we know for sure that his entire source of livelihood is haram, then one should not accept his contribution.
If, on the other hand, the major part of his income is pure and halal, then there is no need to refuse his contribution. We should not shun such a person or discourage him. Perhaps such good deeds can help him to cleanse his income of the taints of Haram: Allah orders His Messenger saying: “Take out charities from their wealth whereby you may purify and cleanse them.” (Qur’an: 9: 103). And the Prophet, peace be upon him, said, “Allah does not wipe out good with bad; He wipes out bad only with that which is good.” (Ahmad)
Having said this, I must also point out: We are to think good of others and should not rush to condemn people for their mistakes; self-righteousness is a deadly sin in Islam.
Allah Almighty knows best.
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