Q: If someone has a fast of Ramadan to makeup, can he make it up on six days of shawwal with two intentions; to get the reward of fasting six days of Shawwal and making up his missed fasts of Ramadan?
In the Name of Allah, Most Merciful and Compassionate
A: According to Imam Ibn Hajar al-Haytami, one obtains the reward of fasting the six days of Shawwal if one intends the sunna fast of Shawwal along with the makeup fast of Ramadan. This is the strongest position in our school. Other scholars of our school, such as Abu Makhrama, said that this is not valid. If you have the resolve, it would be superior to follow the way of caution, which is to fast the makeups and the sunna fasts of Shawwal separately (Bughyat al-Mustarshidin, 113).
And Allah knows best.
Shaykh Hamza Karamali
Since the opinion of Imam al Hafiz Ibn Hajar al Haytami is the strongest position, this should be our answer.