Permissibility of Observing Voluntary Fasts Before Making Up for Days Missed In Ramadan
15-6-2014 | IslamWeb
Question:
What is the ruling on fasting the six days of Shawwaal and the tenth of Muharram for the one who has not yet made up for the days he missed in the previous Ramadan?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that none is worthy of worship except Allah and that Muhammad 
is His slave and messenger.
Muslim jurists have different views concerning the ruling on observing voluntary fasts before making up the days that one has missed in Ramadan. The majority of Hanafi, Maliki, and Shafi‘i scholars maintain that it is permissible to observe voluntary fasts in such a case. However, while the Hanafi scholars hold absolute permissibility, the Maliki and Shafi‘i scholars maintain that this action is permissible, yet unfavorable. The proof of permissibility is that making up for missed fast is not due on an immediate basis.
In contrast, the Hanbali scholars view that it is forbidden to observe voluntary fasts before making up for all the missed fasts of Ramadan, and that if one does so, then his fasting is not valid. The textual evidence by which they support their view is that Abu Hurayrah 
narrated that the Prophet said: “Whoever observes voluntary fast while he still has to make up for days which he missed in Ramadan, his voluntary fast will not be accepted until he makes up for all the days that he had missed in Ramadan.” [Ahmad]
However, according to Ibn Abu Haatim, this Hadeeth is not authentic given the inconsistency of its chains of transmission. Also, the chain of transmission includes Ibn Lahee‘ah as one of the narrators. Additionally, the context of the Hadeeth indicates that there could be rejected parts within the text. For example, the end of the Hadeeth reads: “… and whoever comes to witness Ramadan and has not yet made up for the days of fasting he missed in the previous Ramadan, then his fasting will not be acceptable.” This explanation is mentioned in the book of Al-Sharh Al-Kabeer and is reported by Al-Buhooti in his book entitled Kashshaaf Al-Qinaa’; both books belong to the Hanbali school of Fiqh.
There is another narration that Imaam Ahmad considered it permissible, and even valid, to observe voluntary fast before making up for missed obligatory fasts. In Al-Insaaf, Al-Mardaawi says, “This is the sound opinion.”
Based on this, the preponderant opinion is the one adopted by the majority of Muslim scholars. Consequently, there is nothing wrong with fasting the six days of Shawwaal, the tenth day of Muharram, or any voluntary fast before you observe the days you have missed in the previous Ramadan. This is because there is time to make up for the days missed in Ramadan, while the aforementioned voluntary fasts have a fixed time limit so one should not miss them.
Allah Knows best.