What should be done first: making up the missed days of the month of Ramadan or fulfilling a vow to fast?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

When there is enough time to perform both, it is better to make up the fast of Ramadan before fulfilling a vow to fast. The Hanbali scholars hold the opinion that it is obligatory to make up the fast of Ramadan before fulfilling a vow to fast.

In Al-Mughni, Ibn Qudaamah said:

"When one is required to make up the fast of Ramadan or observe an expiatory fast, he has to perform it before fulfilling a vow to fast. What is made obligatory by the Sharee'ah takes precedence over what is made obligatory by oneself, just as performing the obligatory Hajj takes precedence over the vowed Hajj.

In Al-Insaf, Al-Mardawi said:

"If something that is obligatory by virtue of the Sharee'ah and something that is obligatory by virtue of a vow are simultaneously due (and are of the same type), then one is required to perform what is obligatory by virtue of the Sharee'ah first if there is enough time to observe that which is obligatory by virtue of the vow. If there is not enough time, however, then the vow shall be fulfilled first. When the vow is not specified by a certain time, then making up for a missed act of worship is to be done first."

When there is not enough time to perform both, then making up for a missed act of worship should be done first. For example, if someone failed to fast ten days of Ramadan and vowed to fast ten days of Sha‘ban while only ten days of Sha‘ban remain, then he has to make up for the ten days of Ramadan because the opportunity of fasting them will expire by the end of the month of Sha‘ban. This is the opinion of Ar-Ruhaybani which he gave in Matalib Uli An-Nuha, and he added: "The condition stated by the Hanbali scholars of having enough time necessitates that making up for a missed act of worship must be performed before the vow when there is not enough time for both."

Allaah Knows best.