Combining the intention of making up the missed fast days of Ramadan and the six days of Shawwal
Is it permissible to fast the missed fast-days of Ramadan and the voluntary six days of Shawwal with a single intention?
Many jurists have maintained that it is permissible to subsume a voluntary fast under the intention of an obligatory one, but not vice-versa.
Based on this, it is permissible for a Muslim woman to make up what she has missed of Ramadan during the month of Shawwal. Making up the missed fast-days of Ramadan suffices her for the six days of Shawwal and she obtains their reward since her fast took place during Shawwal. This is analogous to the case of one who enters a mosque and, before sitting down, offers two rak'as with the intention of praying either an obligatory or voluntary prayer by which he obtains the reward of performing the two rak'as of greeting the mosque [Tahiyat-ul-Masjid]. This is because he offered the two rak'as before sitting down. In his meta-commentary, al-Buayrmi stated that the reward is obtained through offering two or more rak'as, whether the prayer was obligatory or another voluntary prayer and whether [the prayer for greeting the mosque] was intended or not because of the words of the Prophet who said, "Whoever enters a mosque should not sit before performing two rak'as" [Bukhari and Muslim]. The hadith denotes that the prayer must be offered before sitting down and this has occurred.
Concerning the issue of fasting, Al-Suyuti stated in Al-Ashbah wa Al-Nadha`ir [p. 22]: "If, for instance, a person fasts on the day of 'Arafa in fulfillment of a make up fast, a vow or expiation, and at the same time makes the intention to fast the day of 'Arafa, his fast is valid according to al-Barzi and he fulfills both fasts." He continued, "The same is true for an undesignated fast by analogy to the case of the prayer for greeting the mosque."
It must be noted that, in this case, the reward obtained is for fulfilling the sunnah of fasting six days during the month of Shawwal though a person will not earn the full reward (tantamount to fasting for an entire year). Al-Ramli wrote in Nihayat Al- Muhtaj (vol. 3, p. 208-9): "If a person observes a makeup fast, a vow or another voluntary fast in Shawwal or if he observes a make up fast on a day such as 'Ashura, he attains the reward of the voluntary fast as my father (Shihab ad-Deen Ahmed ar-Ramli)-may Allah grant him mercy-maintained following the opinion of Barzi, al-Asfuni, al Nashiri, the scholar Ali ibn Salih al-Hadrimi and others, though he will not earn the full reward."
Allah the Almighty knows best.