If you already know it is not from halal source,why would you accept the money in the first place?If he really can't return back your money as there is no halal source,just assume it is sedaqah to him.Money is not everything in life and when we die,we are not gonna take it with us. It is better to be hungry than allow haram money to enter your body.
If the money is in a huge amount and you need that money very badly,you should ask a scholar about this matter