A vow should be fulfilled each time its condition is fulfilled
19-1-2014 | IslamWeb
Question:
I made a vow that I would perform ten Rak‘ahs at Al-Masjid An-Nabawi whenever I commit a certain sin. I used to live in Al-Madeenah then, but I moved to another place that is 200 km from there. I go to Al-Madeenah almost every month, but I have not performed any of the Rak‘ahs, that I vowed to perform, since 1988. Recently, I started to perform as much as I can of those Rak‘ahs in the place where I live. What is your advice to me for I want to annul the vow? Can I perform those Rak‘ahs in the place where I live now?
Answer:
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 
is His slave and Messenger.
Making a vow is a serious matter in the Sight of Allaah The Exalted, and fulfilling it is an obligation that must be fulfilled. Whoever makes a vow that he would do a righteous act that draws him closer to Allaah The Exalted if a certain thing happens is obliged to fulfill his vow and do the righteous act (when that thing happens). The Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever makes a vow to do something in obedience to Allaah should obey Him; and whoever makes a vow to disobey Allaah should not disobey Him.” [Al-Bukhari and others] Allaah The Exalted Underlined in the Noble Quran that fulfilling one's vow is one of the specific attributes of the righteous people that guarantees entering Paradise and escaping Hellfire. Allaah The Exalted Says (what means): {Indeed, the righteous will drink from a cup [of wine] whose mixture is of Kaafoor, * A spring of which the [righteous] servants of Allaah will drink; they will make it gush forth in force [and abundance]. * They [are those who] fulfill [their] vows and fear a Day whose evil will be widespread.} [Quran 76:5-7]
The Prophet, sallallaahu ‘alayhi wa sallam, urged his Companions 
who informed him that they made vows to fulfill them. This happened on many occasions. Therefore, you should fulfill your vow with the condition that you stipulated; that is, to perform the ten Rak‘ahs at Al-Masjid An-Nabawi. If you perform them in any other mosque or place, it would not be sufficient for you nor would it count as fulfillment of your vow. This is because performing prayer in Al-Masjid An-Nabawi is matchlessly meritorious. This is specific to Al-Masjid An-Nabawi and not for any other place except Al-Masjid Al-Haraam. Therefore, as long as you can go to Al-Masjid Al-Haraam and it is easier for you to go there, there is no harm in performing those Rak‘ahs there and thus you would have fulfilled what you vowed and even more.
It is reported that a man stood on the Day of Conquest (of Makkah) and said, "O Messenger of Allaah, I have vowed to Allaah that if He Grants the conquest of Makkah at your hands, I shall pray two Rak‘ahs in Jerusalem. The Prophet, sallallaahu ‘alayhi wa sallam, replied: “Pray here.” The man repeated (his statement) to him and he, sallallaahu ‘alayhi wa sallam, said: “Pray here.” He again repeated (his statement) to him. The Prophet, sallallaahu ‘alayhi wa sallam, replied: “Then, it is up to you.” [Abu Daawood and Ahmad]
Moreover, a woman told Maymoonah 
that she made a vow that she would pray in Jerusalem if she recovered from her illness. Maymoonah said to her, “Stay and eat the provision (which you prepared for your journey) and observe prayer in the mosque of the Messenger, sallallaahu ‘alayhi wa sallam, for I heard him, saying: ‘One prayer in this mosque of mine is better than a thousand prayers observed in other mosque except Al-Masjid Al-Haraam (Mosque of the Ka‘bah).’ [Ahmad and Muslim]
If you are really unable to go to Al-Masjid An-Nabawi or Al-Masjid Al-Haraam, you can perform those Rak‘ahs in your place and you should offer expiation like that of breaking an oath each time your vow becomes due (i.e. whenever you commit a sin). This is because your vow includes doing two acts that draw you closer to Allaah The Exalted; performing a prayer, and performing it in Al-Masjid An-Nabawi. If you are unable to observe one of them, you should do the one you can observe and offer expiation like that of breaking an oath for failing to do the other. The Prophet, sallallaahu ‘alayhi wa sallam, said: “If anyone takes a vow he is unable to fulfill, its expiation is the same as that for an oath, but if anyone takes a vow he is able to fulfill, he must do so.” [Abu Daawood and Ibn Maajah]
In fact, you cannot annul the vow, but you have to fear Allaah The Exalted and avoid committing sins. It would be forbidden for you to commit sins even if you had not made that vow. You should also seek forgiveness from Allaah The Exalted, turn to Him in repentance from past sins, and implore Him to Safeguard you from committing sins in what is left of your life. Indeed, Allaah The Exalted Is Accepting of repentance, All-Forgiving, Ever-Merciful, Generous and Noble.
Allaah Knows best.