it is all the more permissible and appropriate to buy from a Muslim what he has received under a contract he believes permissible—even if we consider it forbidden. For a non-Muslim’s interpretation that conflicts with the religion of Islam is unquestionably invalid, unlike a Muslim’s interpretation.

Thus, if they embrace Islam and bring their disputes before us, having received wealth through contracts they believed permissible—such as interest payments or the proceeds from selling wine and swine—that wealth does not become forbidden to them. Likewise, dealing with them in that wealth before their conversion is not forbidden. This is based on the words of the Most High:

“Fear Allah and give up what remains of interest.”

He did not forbid them what they had already received. The same applies to someone who engaged in interest-based transactions believing them permissible, then discovered that they were not permissible, provided these were transactions over which Muslims have differed. According to the sound position, what he received through those transactions is not forbidden to him.

The second consideration:

Whatever rulers have taken through outright injustice, if it has become mixed with the funds of the public treasury and cannot be returned to its owner, is to be spent on the public interests of the Muslims. For what is unknown is treated as nonexistent. Thus, property known to have been taken unjustly, but whose owner is unknown, is to be spent on public interests.

Whatever is received from a public treasury in which lawful and unlawful funds are mixed is not to be deemed forbidden. For when property becomes indistinguishably mixed, this is treated as equivalent to its destruction, and its owner is entitled to compensation from the public treasury. Therefore, according to the soundest scholarly position, it is permissible to receive payment for something sold from the mixed funds of the public treasury. And Allah knows best.