What is the ruling in the Maliki school regarding the consequences for me if I break a vow to abstain from a sin by fasting three days, and does fulfilling the vow entail fasting as I obligated myself, or is it an expiation for an oath?

A "nathr al-lajaj" (oath of contention/obstinacy) is a vow made with the intention of preventing oneself from doing something or punishing oneself, such as saying, "It is a vow upon me for the sake of Allah if I speak to so-and-so." The Maliki school of thought differs on its ruling. The well-known opinion is that it is obligatory to fulfill it, based on the hadith, "Whoever vows to obey Allah, let him obey Him." Some, however, hold that an expiation for an oath (kaffarat yamin) is sufficient, relying on the hadith, "The expiation for a vow is the expiation for an oath." If the one who made the vow fasts what he vowed, that is sufficient for him, even according to the view that requires expiation.