Is failing to perform Sunnah prayers out of necessity considered a breach of a vow, necessitating expiation, and must I offer expiation every time I omit a Sunnah prayer?

A vow contingent upon a disliked condition may even reach the level of prohibition, yet it must be fulfilled if the condition is met. If you vowed to perform supererogatory prayers (Sunan), you are obligated to observe them. If a necessity arose that prevented you from performing a supererogatory prayer you vowed, scholars have differed: The Malikis do not obligate its making up due to an excuse, but they do obligate it if intentionally abandoned. The Shafi'is obligate its making up even with an excuse. The Hanbalis differentiate between temporary inability, which is expected to cease, in which case one waits, and continuous inability, in which case expiation is required. If expiation becomes necessary, then for each missed supererogatory prayer, there is an expiation.