Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this Fatwa:
1- As a priority, the first option is to always make the obligatory actions up first.
2- The second option is that let’s say you had many fasts to make up, like twenty, and you started on the 10th of Shawwal. So, once you finish the obligatory, you are going to be outside of Shawwal. So when are you going to do the six days of Shawwal? Well, what you would do there is that you would actually do the obligatory fasts, and then you would follow it up with six extra with the intention of it being Shawwal even though you are not in Shawwal.
3- The third option is that if you have many missed fasts, you can begin with the six days of Shawwal and you can make up the missed fasts later.
Sheikh Abu Eesa Niamatullah answers these questions in this Video:
Q: Can I begin to fast my 6 days of Shawwal before making up my missed fasts in Ramadan? We are now in the month of Shawwal and most us of aware of the hadith mentioning fasting the 6 days of Shawwal. However, what becomes a priority when you have fasts that you have missed this past Ramadan? Should you make those up first, or is it okay to begin with the 6 days of Shawwal before making up the obligatory fasts you've missed?Support Faith IQ here: https://www.launchgood.com/project/faith_iq#/
Posted by Faith IQ on Tuesday, 27 June 2017
Source: Faith IQ Facebook page.
Allah Almighty knows best.