as salam Aleikum brother and sisters

would it be haram to work with a non - muslim/muslim who earns haram money?

whats the difference , if someone wants to buy something from me do i need to look to look at their background to find out if the money is haram or halal? for example if a white person buys an item from me , i usually presume their money would be mixed with clean and haram money , i have doubts , help me please thank you

its been on my mind for a long time