What is the ruling concerning the salary of my colleague who works in an association that deposits its money in a bank and spends from its interest, knowing that he does not know whether his salary is from this interest or if it is mixed, and should the unlawful portion of the mixed salary be estimated and disgorged?

If a person's work in the association does not involve usurious transactions, there is no impediment to his working there and receiving his salary from it, even if the association deposits its funds in usurious banks. The sin falls upon the association if it takes usurious interest. Scholars have permitted dealing with those whose lawful wealth is mixed with unlawful wealth if the majority of their wealth is lawful, and they differed concerning those whose majority of wealth is unlawful. The basic principle is that there is no obligation to investigate the source of wealth, as the Prophet, peace and blessings be upon him, dealt with Jews and ate from their gifts without inquiry.