Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
If you have no evidence to prove he has earned money in a haram way, you are allowed to accept it. Everyone is innocent until proven otherwise.
Answering your question, Sheikh Ahmad Kutty, a Senior Lecturer and an Islamic Scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
If you know for sure he is returning your money by using the money he earned from haram sources, you should not accept it; however, you are not supposed to act on mere suspicion.
If you have no evidence to prove he has earned money in a haram way, you are allowed to accept it. Everyone is innocent until proven otherwise.
Read also:
Can I Pay Off My Debt In a Different Currency?
Can We Give Zakah to Prisoners in Debt?
Can I Deduce My Debt from My Zakah?
Is Zakah Due on Properties That Are in Debt?
Is There Zakah on the Debt Someone Owes You?
Allah Almighty knows best.