What is the disagreement among scholars regarding the nullification of an apostate's deeds if he returns to Islam? Do his righteous deeds remain or are they nullified? Is this a matter related to the will of Allah?

The scholars have agreed on the acceptance of the apostate's repentance and its benefit to him if it is sincere, and that Allah replaces his evil deeds with good ones, as stated in verse 70 of Surat Al-Furqan. The preferred view is that the apostate's deeds are not nullified unless he dies upon disbelief, based on Allah's saying in Surat Al-Baqarah, verse 217: "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." This is the madhhab of Ash-Shafi'i and others, where the nullification is restricted to death upon disbelief, and the unrestricted is interpreted as restricted. Furthermore, the apostate may be rewarded for the good deeds he performed during his apostasy if he dies as a Muslim. As for the whispers of disbelief, they are mostly unfounded and are a form of waswas (obsessive whispering), and resisting them is a struggle for which one is rewarded, and its cure is to disregard them.