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Haraam wealth is of two types:
So if the wealth is haraam in and of itself, it is not permissible for him to pay off the debt with it, and it is not permissible for you to accept it.
This wealth is haraam only for the one who acquires it, according to some scholars, and it is not haraam for the one to whom it is passed on by permissible means, such as a gift or maintenance.
See the answer to question no. 289442.
If the one who acquired this wealth has repented, and is in need, it is permissible for him to take from it as much as he needs, and it is permissible for him to pay off his debt with it.
Based on that, if the debtor does not own anything except this haraam wealth, there is nothing wrong with him paying off his debt with it, and there is no blame on you for accepting that.
What appears to be the case from the question is that he does not currently possess this haraam wealth; rather he wants to commit sin in order to pay off his debts. If that is the case, according to what we understand, then it is not permissible for him to do that, and it is not permissible for you to call on him to do that. Rather you must give him more time, if he is indeed in financial difficulty, because Allah, may He be exalted, says (interpretation of the meaning): “And if someone is in hardship, then [let there be] postponement until [a time of] ease” [al-Baqarah 2:280].
Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) said: One of the things we learn from this verse is that it is obligatory to give one who is in difficulty more time, until he becomes well-off, because Allah, may He be exalted, says: “then [let there be] postponement until [a time of] ease.” So it is not permissible to demand that he pay off the debt or to ask him for what is owed.
End quote from Tafseer Soorat al-Baqarah (3/391).
And Allah knows best.