What is the ruling on someone who utters "may Allah curse the religion of" in a moment of anger without completing it? And what is the difference between the nullification of a deed and the nullification of the reward for a deed, and does an apostate, if they repent, retain their deeds while losing their reward?

Firstly: Cursing the religion is apostasy and disbelief. As for someone who says "God curse the religion" (or "damn the religion") without completing the phrase, their ruling is not the same as someone who curses the religion entirely. They should seek God's forgiveness and control their anger.

Secondly: The difference between the nullification (ḥubūṭ) of a deed (ʿamal) and the nullification of its reward (thawāb) is that the nullification of a deed means its invalidation, which necessarily entails the nullification of its reward. However, the nullification of the reward means not receiving it despite the deed being valid. Examples include praying in a usurped house, or for someone who visits a diviner, or for a perfumed woman going to the mosque. The prayer of these individuals is valid and fulfills the obligation, but they receive no reward for it.

Thirdly: When an apostate repents, their previous good deeds performed during their time as a Muslim are not nullified, because the nullification of deeds requires dying upon disbelief.