Is a man's Hajj valid and sufficient for him as Hajjat al-Islam if he performed it, then became negligent in prayer and abandoned it, then repented?

The Hajj performed by someone who is an unbeliever does not suffice for the Hajj of Islam. However, if a person was a Muslim, performed Hajj, then apostatized and subsequently repented, their Hajj remains valid. This is because they performed it while being a Muslim. The deeds of an apostate are not nullified unless they die upon disbelief, as Allah Almighty says: (And whoever of you reverts from his religion and dies while he is a disbeliever - for those, their deeds have become worthless in this world and the Hereafter, and those are the companions of the Fire; they will abide therein eternally.) Al-Baqarah/217.