Is trading foreign currencies permissible or forbidden if their source, whether legitimate or illegitimate, is unknown and cannot be verified?

Working in currency exchange is permissible if the Sharia regulations are observed to avoid usury (riba). As for trading in smuggled currencies, it is not permissible if it causes harm to the country and its people, due to the infliction of harm and cooperation in sin and aggression. One can inquire about how currencies enter from those who are knowledgeable in this matter.