What is the ruling on receiving payment from a person who deals with a Riba-based bank after advising him to deal with an Islamic bank, and what is the ruling on dealing with him (such as buying from him or using his belongings) without knowing that he deals with Riba?

If working with the aforementioned individual has no relation to the forbidden (haram), then there is no harm in it. The evidence for this is the Prophet's—peace and blessings be upon him—approval of Ka’b ibn Ujra’s act of providing water to a Jew in exchange for dates. However, if the work itself is forbidden, then it is not permissible. Regarding buying and selling with someone whose wealth is entirely or partially illicit, if one is unaware of this fact, there is no sin.