Is it permissible to accept the repayment of a debt from money that is suspected of being Haram, if refusing it might lead to the loss of the money?

There is no objection to accepting debt repayment from someone whose money has a taint of unlawfulness (shubhāt al-haram). Unlawful money is either unlawful in itself, like stolen money, which is impermissible to take possession of, or unlawful due to its acquisition, like usury. The sin of the latter falls upon the earner, and it is permissible for someone who acquires it through a permissible means to benefit from it. The Prophet, peace and blessings be upon him, dealt with the Jews, whom Allah described as consuming usury. Money that has a taint of unlawfulness (shubhāh) has a lighter ruling, and there is no harm in accepting it as debt repayment. The sin is upon the one who acquired it. Sheikh al-Islam Ibn Taymiyyah mentioned that taking possession of money with a taint of unlawfulness is not forbidden; rather, abstaining from it is a recommended piety (wara' mustahabb), and avoiding what is doubtful is not obligatory.