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:
The Standing Committee for Islamic Research and Iftaa’ issued the following fatwa:
First, it is better not to observe voluntary fasts when you still owe one or more days from Ramadan. Rather you should start with the fasts that you owe from Ramadan, and then observe voluntary fasts.
Second, if you fast the 10th and 11th of Muharram with the intention of making up for days that you owe from Ramadan, then it is permissible and will make up for two of the days that you owe.
The Prophet (peace and blessings be upon him) is reported to have said, “Actions are judged but by intentions, and everyone shall have but that which he intended.” (Al-Bukhari)
We hope that you will have the reward for making up the missed fast and the reward for fasting that day.
Almighty Allah knows best.