Praise be to Allah.
Making a promise to Allaah is a kind of vow (nadhr). See question no. 38934.
Allaah has commanded us to fulfil our promises and covenants, whether the promise is made to Allaah or to people. Allaah says (interpretation of the meaning):
“And fulfil the Covenant of Allaah (Bay‘ah: pledge for Islam) when you have covenanted”
“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”
This applies if what the person wants to do is an act of worship, such as when a person makes a promise to his Lord that he will speak the truth or give some of his wealth in charity, etc. This comes under the heading of vows (nadhr) even if the person does not actually use the word nadhr or vow. The one who makes a promise to Allaah or says, “I promise Allaah that I will do such and such” is obliged to fulfil it, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever vows to do an act of worship or obedience to Allaah, let him fulfil it, and whoever vows to do an act of disobedience towards Allaah, let him not fulfil it.” Narrated by al-Bukhaari, 6318.
It is not permissible to fail to fulfill this obligation, unless a person is unable to fulfil it at all, in which case he should offer kafaarat yameen (expiation for breaking a vow), because of the hadeeth: “Whoever makes a vow and is unable to fulfil it, his expiation is kafaarat yameen.” Narrated by Abu Dawood, 3322, from the hadeeth of Ibn ‘Abbaas. Al-Haafiz said in al-Fath: Its narrators are thiqaat, but it was narrated by Ibn Abi Shaybah in a mawqoof report, which is likely to be the case.
Ibn Qudaamah said in al-Mughni (10/72): In conclusion: the one who vows to do an act of worship but is unable to do it, or was able to do it then became unable to do it, has to offer kafaarat yameen.
But if what the person wants to do is a sin, then it is not permissible for him to fulfil his vow, such as if a person makes a promise to his Lord that he will not speak to his brother or visit him, when there is no shar’i justification for that. The Prophet (peace and blessings of Allaah be upon him) said, according to the hadeeth quoted above: “and whoever vows to do an act of disobedience towards Allaah, let him not fulfil it.”
What is required in this case is repentance to Allaah from making a promise to commit sin, and offering kafaarat yameen, because the Prophet (peace and blessings of Allaah be upon him) said: “There is no vow to commit sin, and its expiation is kafaarat yameen.” Narrated by Ahmad (2640); Abu Dawood (3290); al-Tirmidhi (1524); al-Nasaa’i (3834); Ibn Maajah (2125). Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
Similarly if a person makes a promise to Allaah to do something, then he sees that something else is better, such as if he wants to not treat a relative or friend well, or not spend on him, then he should not fulfil that, rather he should refrain from it, whether that was a vow or an oath. The Prophet (peace and blessings of Allaah be upon him) said: “Whoever swears an oath then sees that something else is better than it, let him do that which is better and let him offer expiation for his oath.” Narrated by Muslim, 1650, from the hadeeth of Abu Hurayrah (may Allaah be pleased with him).
The questioner does not mention the promise that she made so that we can answer in more specific terms, but she can learn the ruling on what she has done from what we have said above.
And Allaah knows best.