[Dealing with One Who Engages in Usury]

The answer—and Allah is the granter of success and aid—is that it is permissible to deal in buying and selling with the usurer and the oppressor—except in what is known specifically to be unlawful in itself; for example, a thief offers a car for sale and you know it is stolen. In such a case, it is not permissible to purchase it.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2