What is the ruling on a person who embraced Islam using money and property he acquired through unlawful means, according to Islamic law, when he was a disbeliever? And does this count as an endorsement of seeking judgment from other than Allah's law?

It is not permissible to take another person's property without right, whether by theft, usurpation, embezzlement, or aggression, due to the Almighty's saying: "And do not consume one another's property unlawfully," and the Prophet's (peace be upon him) saying: "Indeed, your blood, your properties, and your honors are sacred to you." Whoever takes something without right must return it to its owner, due to the Prophet's (peace be upon him) saying: "On the hand is what it has taken until it returns it." It is not permissible for him to dispose of it. Among the conditions of repentance is returning rights to their owners, either directly with a request for forgiveness, or indirectly if direct interaction is not possible.