Firstly: 

Vows are of two types: 

1 – Conditional vows, in which the vow is dependent upon something happening, as when a person says, “If Allaah heals me, I will give such and such in charity, or I will fast such and such a number of days,” and so on. 

2 – Unconditional vows (which are not connected to anything), such as saying, “I will fast such and such a number of days for Allaah.” 

Both types of vows must be fulfilled, if what is vowed is to do an act of worship. 

The Prophet (peace and blessings of Allaah be upon him) said: “Whoever vows to obey Allaah, let him obey Him, and whoever vows to disobey Him let him not disobey Him.”  

Narrated by al-Bukhaari, 6696. 

The obligation to fulfil a vow is stronger in the case of an conditional vow, although it is obligatory to fulfil both types, as stated above. 

Ibn al-Qayyim said: 

If a person says: “If Allaah delievers me I will give in charity,” then this is a promise that he has made to Allaah and that he must fulfil, otherwise he will be included in the group mentioned in the verse (interpretation of the meaning): 

“So He punished them by putting hypocrisy into their hearts till the Day whereon they shall meet Him, because they broke that (covenant with Allaah) which they had promised to Him and because they used to tell lies”

[al-Tawbah 9:77]

The promise that a person makes to his Lord is a vow that he must fulfil. The conditional vow is even more essential to fulfil than a vow in which a person says, “I will do such and such for Allaah.” 

Breaking it will result in the punishment of hypocrisy in the heart. 

End quote. 

Secondly: 

The Prophet (peace and blessings of Allaah be upon him) condemned those who made vows but did not fulfil them. Muslim (2535) narrated from ‘Imraan ibn Husayn that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The best of you are my generation, then those who follow them, then those who follow them, then those who follow them.” ‘Imraan said: I do not know whether the Messenger of Allaah (peace and blessings of Allaah be upon him) said, “and those who follow them” two times or three. “Then after them will come people who will bear witness without being asked to do so, who will betray and not be trustworthy, who will make vows but not fulfil them, and fatness will appear among them.”  

Al-Nawawi said: 

This shows that it is obligatory to fulfil vows, and there is no dispute among the scholars concerning that.  

End quote. 

What is meant by the words of the Prophet (peace and blessings of Allaah be upon him), “and fatness will appear among them” is that these people will be negligent towards their religious affairs, and all they will care about is eating, drinking, relaxing and sleeping. The kind of fatness that is condemned is that which is acquired, not that which is the way a person is made. And Allaah knows best. See ‘Awn al-Ma’bood, commentary on hadeeth no. 4657. 

Thirdly: 

Not fulfilling vows is one of the attributes of the hypocrites. 

Allaah says (interpretation of the meaning): 

“And of them are some who made a covenant with Allaah (saying): ‘If He bestowed on us of His Bounty, we will verily, give Sadaqah (Zakaah and voluntary charity in Allaah’s Cause) and will be certainly among those who are righteous.’

Then when He gave them of His Bounty, they became niggardly [refused to pay the Sadaqah (Zakaah or voluntary charity)], and turned away, averse.

So He punished them by putting hypocrisy into their hearts till the Day whereon they shall meet Him, because they broke that (covenant with Allaah) which they had promised to Him and because they used to tell lies.

Know they not that Allaah knows their secret ideas, and their Najwa (secret counsels), and that Allaah is the All-Knower of things unseen”

[al-Tawbah 9:75-78]

Fourthly: 

With regard to the punishment of one who does not fulfil the promise he made to Allaah: 

There is the fear that Allaah may punish him by instilling hypocrisy in his heart, so he will meet Allaah as a hypocrite and will be one of the losers.

As Allaah says in the verse quoted above (interpretation of the meaning): 

“So He punished them by putting hypocrisy into their hearts till the Day whereon they shall meet Him, because they broke that (covenant with Allaah) which they had promised to Him and because they used to tell lies”

[al-Tawbah 9:77]

al-Sa’di said (p. 546): 

This means: among these hypocrites are those who made a covenant with Allaah and said, “If He bestowed on us of His Bounty” in this world, granting us a life of ease and plenty, then “we will verily, give Sadaqah (Zakaah and voluntary charity in Allaah’s Cause) and will be certainly among those who are righteous” and will uphold the ties of kinship and support the truth and do good deeds. 

Then “when He gave them of His Bounty”, they did not keep their promise, rather they “they became niggardly [refused to pay the Sadaqah (Zakaah or voluntary charity)], and turned away” from obedience and submission and were “averse” i.e., they had no interest in doing good. When they did not fulfil the promise that they had made to Allaah, He punished them by “putting hypocrisy into their hearts” which will last “till the Day whereon they shall meet Him, because they broke that (covenant with Allaah) which they had promised to Him and because they used to tell lies”. 

So let the believers beware of this evil characteristic whereby a person promises his Lord that if a certain hoped-for thing happens, he will do such and such, then he does not fulfil that promise, for Allaah may punish him with hypocrisy just as He punished the people mentioned here. 

The Prophet (peace and blessings of Allaah be upon him) said, according to the hadeeth proven in al-Saheehayn: “The signs of the hypocrite are three: when he speaks, he lies; when he makes a covenant, he betrays it; and when he makes a promise, he breaks it.” 

This hypocrite who made a promise to Allaah that if Allaah gave him of His bounty he would give charity and be among the righteous, spoke and lied, make a covenant and betrayed it, and made a promise and broke it. Hence those who do such evil actions are addressed with the words (interpretation of the meaning): 

Know they not that Allaah knows their secret ideas, and their Najwa (secret counsels), and that Allaah is the All-Knower of things unseen”

[al-Tawbah 9:78]

Allaah will recompense them for their deeds which are known to Him. 

And Allaah knows best.