Existence of a vow is contingent upon the existence of its reason
5-8-2015 | IslamWeb
Question:
My sister vowed to spend her first salary in charity throughout the year, giving a part of the salary in charity each month. Allah, the Exalted, blessed her with a job and she started fulfilling her vow for two months, but now she has the chance to complete her studies (and will have to quit her job). If she willingly quit her job in order to complete her studies, should she pay the rest of the vowed money or postpone it until she gets another job?
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.
The vow-taker is obliged to fulfill his vow in the very manner that he intended it when he took his vow. This is because ‘Aa’ishah 
reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to be obey Allaah, the Exalted, must obey Him..." [Al-Bukhari] Your sister did well by hastening to fulfill her vow when she got the job. By quitting her job, your sister's vow is suspended until she gets another job. When she gets another job, she should continue to fulfill her vow and pay the rest of the vowed money. The vow ceases to exist when its reason ceases to exist, i.e. getting a job. Therefore, the preponderant opinion is that it is not incumbent on her to give the rest of the vowed money in charity except after she gets another job. We would like to point out that scholars considered conditional vows disliked because they do not bring about any good, neither do they ward off any harm or repel a divine decree. On the contrary, they may cause hardship and difficulty, which might lead to the non-fulfillment of the vow. Ibn ‘Umar 
related that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, forbade them from making vows and said: "It does not bring good; it is just a means of taking wealth from the miserly person." [Al-Bukhari, Muslim, and others]
Allaah Knows best.