What is the ruling on fulfilling a vow (Nidhr) related to an act that is not considered obedience (Ta'ah)?

Primary text

If a vow is made concerning an act that is not an act of obedience, only the obedience portion of the vow must be fulfilled. The Prophet, peace be upon him, commanded Abu Isra'il to complete his fast and abandon the rest because the abandoned act was not one of obedience. The obligation of expiation for the abandoned non-obligatory part is subject to differing scholarly opinions previously mentioned.