How is his saying, may God’s prayers and peace be upon him, in the Hadith (If He wills, He forgives him, and if He wills, He punishes him) reconciled with the fatwa that declares whoever abandons prayer out of negligence to be a disbeliever, given that God does not forgive a disbeliever?

The Hadith (Five prayers Allah has obligated...) was narrated by Abu Dawud and Ahmad and authenticated by Al-Albani. The Prophet's (peace be upon him) statement ("If He wills, He will forgive him") does not mean that one who abandons prayer does not disbelieve (kafir). This has been interpreted as referring to someone who did not perform the prayer completely, with its ablution, bowing, and humility, or to someone who prays sometimes and abandons it sometimes, and not to someone who abandons it entirely. Sheikh al-Islam Ibn Taymiyyah clarified that not maintaining prayer means performing it after its due time, not abandoning it entirely.