Is the voluntary fast that the questioner previously observed sufficient to fulfill the obligatory fast that she has not yet made up, should she die before completing the days she owes?

If a servant's obligatory duty is deficient on the Day of Judgment, whether in prayer or fasting, it will be completed from their voluntary (supererogatory) acts, as stated in the Hadith (definition) of Abu Hurairah (may Allah be pleased with him) found in the Musnad and Sunan collections. Scholars have differed regarding the meaning of a deficient obligatory duty. Some are of the view that it refers to a deficiency in the Sunan (prophetic traditions) and the mannerisms (of worship), while others believe it refers to abandoning the obligatory duty entirely. It is incumbent to strive in fulfilling obligatory duties and not rely on voluntary acts. Ibn Abd al-Barr stated that the completion of an obligatory duty from voluntary acts is specific to one who forgot it, not to one who intentionally abandoned it or remembered it but did not perform it. The days that were fasted with the intention of making up (missed fasts) and the six days of Shawwal will count as making up the missed fasts, and it is hoped that the reward for fasting the six days will also be attained.