Can I Fast Ashura Before Making Up Missed Ramadan Fasting?
Question

I still have days to make up for Ramadan and I want to fast 'Ashura’. Is it permissible for me to fast 'Ashura before I have made up the fasts I owe? Can I fast 'Ashura’ and the eleventh of Muharram with the intention of making up the Ramadan fasts, and will I get the reward for fasting 'Ashura’?.
Praise be to Allah.Firstly: 
You should not 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. 
Secondly: 
If you fast the tenth and eleventh of Muharram with the intention of making up days that you owe from Ramadan, that is permissible and will make up for two of the days that you owe. The Prophet (peace and blessings of Allah be upon him) said, “Actions are judged but by intentions, and every person will have but that which he intended.”  Fatawa al-Lajnah al-Daa’imah, 11/401 
We hope that you will have the reward for making up the missed fast and the reward for fasting that day. 
Fatawa Manaar al-Islam, by Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him), 2/358.
For more information, see the answers to questions No. (21775 ) and (303756 )
And Allah knows best.