Making a vow contingent on the will of Allaah and ruling on delaying the fulfillment of a vow
3-8-2015 | IslamWeb
Question:
Before I gave birth, I supplicated Allah to have my child healthy and fully formed. I also said: "If Allah wills, I will pray 100 Rak‘ahs (units of prayer) at once for the His sake immediately after the end of the post-partum period." I, however, failed to fast immediately after the post-partum period. Also, I could not pray them all at once. Can I pray them separately? What should I do for failing to pray the vowed Rak‘ahs immediately after the post-partum period? How can I fulfill my vow, which I do hope to fulfill?
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, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Making a vow is disliked in the Sharee'ah due to the relevant forbiddance. A wording that implies a pledge (such as saying "It is due to Allaah upon me to do such-and-such," or, "I vow to do such-and-such.") is one of the pillars necessary for rendering the vow binding. In Al-Mughni, Ibn Qudamah 
said: "The wording of the vow is to say, 'It is due to Allaah upon me to do such-and-such'. Also, when one says, 'I vow to do such-and-such', then the vow is considered binding because he explicitly expressed its wording. If he said, 'I will fast a month if Allaah heals me,' then that is also considered a vow. If he said, 'It is due to Allaah upon me that I walk to the House of Allaah', (then know that) Ibn ‘Umar considered that a vow and that he is required to walk."
Consequently, if you made a similar wording, the vow is binding. If you attached the clause "if Allaah wills" to the statement of the vow, then you are not required to fulfill it, according to most of the scholars.
But if you did not attach this clause to the statement of the vow or did not mention it at all, then you are required to fulfill the vow at the specified time. Nevertheless, most of the scholars hold the opinion that the expiation is not due in case of delaying the fulfillment of a vow. Nonetheless, you are still required to fulfill the vow according to the way you specified. Performing those Rak‘ahs separately does not clear you of your liability. You can wait until you are able to fulfill them and then do so. If you are permanently unable to fulfill the vow, then you are required to offer an expiation like that for breaking an oath only.
If you did not say the formula of a vow but you only said a statement signifying a promise, then you are not obliged to fulfill it, but it is recommended to do so.
Allaah Knows best.