What is to be done with the wealth left behind by somebody who died and it is not known for sure whether he was a Muslim or not? It may be that it was unknown as to whether he became a Muslim or not before his death or it may be that there was a difference of opinion about whether such a person is to be considered a Muslim or not (such as one who only prayed Friday prayer and occasionally other prayers, but not always observing them at the proper time, or it may be thought that their 'Aqeedah was corrupt but it may not be known for sure? Should the Muslim heirs refrain from taking anything of the wealth he left behind to be on the safe side (in case the person died as a kaafir)? Should Hajj be done on behalf of the dead person (in case he died as a Muslim)?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If you mean that the person who you referred to in the question is a Muslim by origin, as one born to Muslim parents, and not someone whose religion is not known, and it is suspected that he might have apostatized before his death, then he is considered a Muslim and his Muslim heirs inherit from him. Such a person is not to be declared a non-Muslim based only on suspicion of his apostasy. If one is a Muslim with certainty, he cannot be declared an apostate based on suspicion. It is impermissible to declare a specific Muslim an unbeliever just because he has committed an act of unbelief. He may have committed that act out of ignorance and so would be excused, and he may have committed it due to a faulty interpretation or compulsion. These reasons (ignorance, faulty interpretation and compulsion) prevent declaring someone who commits an act of unbelief an unbeliever as highlighted in Fatwa 15255.

Ibn Taymiyyah said,

“No one is entitled to declare a Muslim an unbeliever, even if he or she commits an act of unbelief unless proofs are established against him or her beyond any excuse. If one is a Muslim with certainty, he cannot be declared an unbeliever by suspicion only; rather, proofs should be established against him and all misconceptions and doubts must first be removed.”

He also said,

“Labeling a specific Muslim individual an unbeliever and declaring him a permanent dweller of Hell depends on the fulfillment of the prerequisites for declaring unbelief and the absence of all its impediments.”

We previously highlighted in Fatwa 90899 that a Muslim who abandons prayer out of laziness is not to be declared an unbeliever according to the majority of the scholars, and can inherit from his Muslim relatives and his Muslim heirs can inherit from him.

As for the ruling on performing Hajj on behalf of the dead person in case he died as a Muslim, please refer to Fatwa 96372

Allaah Knows best.