What is the ruling on swearing with phrases such as "By God, I will not forgive him" or "By God, you will make it a black day," whether intended as a threat or not?

The oath that is considered valid is that which its taker intends and purposes. As for the oath uttered without intention, it is considered a 'laghw' (idle oath) for which there is no expiation, based on the Almighty's saying: (لَا يُؤَاخِذُكُمُ اللَّهُ بِاللَّغْوِ فِي أَيْمَانِكُمْ وَلَكِنْ يُؤَاخِذُكُمْ بِمَا عَقَّدْتُمُ الْأَيْمَانَ) "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what your hearts have intended," and His saying: (وَلَيْسَ عَلَيْكُمْ جُنَاحٌ فِيمَا أَخْطَأْتُمْ بِهِ وَلَكِنْ مَا تَعَمَّدَتْ قُلُوبُكُمْ) "And there is no blame upon you for that in which you have erred, but [blame is due for] what your hearts have intended."

Scholars have differed in defining the idle oath. Ibn Abbas holds that it is a man's saying "No, by Allah" or "Yes, by Allah" in the course of conversation without intending an oath. Aisha, may Allah be pleased with her, holds that idle oaths are those uttered in argumentation, jest, play, and speech upon which the heart does not firmly resolve.

As for the intended and deliberate oath, expiation becomes due for it, whether the intention was to commit to it or to threaten. So, whoever swears an oath over something and then breaks it, must offer expiation for the oath, which is feeding ten poor people, or clothing them, or freeing a believing slave. And whoever cannot find [the means], then [he must] fast for three days.