Is it permissible for me to ask the debtor about the source of the money with which he will repay his debt, to ensure it is not from usury, theft, or anything unlawful?

You are not required to search for the source of the funds that will be paid to you, and being unaware of it does not harm you. The default is safety and good faith. However, if you know that what was given to you is precisely haram (unlawful), then it is not permissible to take it.