What is the ruling on someone who swore with the صيغة (formulation) "By the kaffara and a year's fasting, I will not do that thing," regarding a specific matter, and then did it, whether inadvertently or intentionally?

A person's statement: "By atonement and fasting for a year, I will not do such and such," is not considered a legally binding oath nor a vow, due to the absence of anything indicating commitment. However, if he says: "I must fast for a year if I do such and such," intending to prevent himself, this is called a vow of obstinacy and anger (nadhir al-lajaj wal-ghadab). In this case, there is a choice between not doing the mentioned thing (and nothing is due from him), or doing it and then choosing between fasting for a year or offering an expiation for an oath (kaffarat yameen). This is the view of the Hanbalis and was chosen by Al-Nawawi from the Shafi'is.

According to the Malikis, he is obligated to fast for a full year if he made such a vow and then broke it. Whoever breaks an oath intentionally is liable for expiation. Whoever breaks an oath out of forgetfulness is not liable for expiation according to the Shafi'is and one narration from the Hanbalis, while he is liable according to the Hanafis and Malikis, unless the Malikis qualify this by stating: "provided he did not say: 'unless I do the sworn-upon act forgetfully'."