Fast what is upon you from missed fasts first
Is it permissible to fast the 6 days of Shawwaal before making up the missed fasts of Ramadhan? And is it permissible to fast on Monday in the month of Shawwaal with the intention of making up the missed fasts of Ramadhan, and with the intention of attaining the reward of fasting on the Monday?
The reward of fasting the six days of Shawwaal is not attained except if a person has completed the month's fasts of Ramadhan, so whoever has some missed fasts from Ramadhan then let him not fast the six days of Shawwaal until he has completed the missed fasts of Ramadhan, because the Prophet sallallaahu 'alaihi wasallam said:
ãä ÕÇã ÑãÖÇä Ëã ÃÊÈÚå ÈÓÊ ãä ÔæÇá
Whoever fasts Ramadhan then follows that with six from Shawwaal. [Muslim, Abu Dawud, Tirmidhi, Ibn Majah, Ahmad, ad-Daarimi].
So based upon this, we say to anyone who has missed fasts, upon him is that he must fast those first, then fast the six days of Shawwaal. And if it happens that these six days fall upon a Monday or Thursday then indeed he will attain the reward of them both [the six days and the fasting upon the Monday or Thursday], with the reward of the intention of fasting the six days and with the reward of the intention for fasting Monday or Thursday, due to his sallallaahu 'alaihi wasallam's saying:
ÅäãÇÇáÃÚãÇá ÈÇáäÊÇÊ æÅäãÇ áßá ÇãÑÆ ãÇ äæì
Actions are judged by their intentions and for every person is that what he intended
Shaykh Muhammad bin Saalih al-`Uthaymeen
Fiqhul Ibaadaat Page No.222 Maktabatul Iman