Making a vow for worldly matter

Making vows in Islam so they are to be fulfilled if Allah answered what you asked from Him in a Du'a, is permitted only if the vows are about any act of worship or obedience to Allah that are not obligatory. Praying the five prayers or fasting in Ramadan are both obligatory worshipping while praying extra Rakat is not. On the other hand, the vows that are to do any worldly benefit are disliked (Makrooh) like charity doings.

The quote which supports that (also mentioned in @questionmark123's answer):

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:

The basic principle is that making vows is makrooh (disliked), because it is proven that the Prophet (blessings and peace of Allah be upon him) disallowed making vows and said:

"It does not bring any good; it only gets something out of the miser."

Majmoo‘ al-Fataawa (35/354)

Note that this is different from doing worshipping for Allah that I've mentioned first, and here is an example from Qur'an which proves that such vows are allowed in Islam. In Surah Al-Insan (76):

Verse 5:

In Arabic (the exact wording):

إِنَّ الأَبْرَارَ يَشْرَبُونَ مِن كَأْسٍ كَانَ مِزَاجُهَا كَافُورًا

The translation in English:

Indeed, the righteous will drink from a cup [of wine] whose mixture is of Kafur.

Verse 6:

In Arabic (the exact wording):

عَيْنًا يَشْرَبُ بِهَا عِبَادُ اللَّهِ يُفَجِّرُونَهَا تَفْجِيرًا

The translation in English:

A spring of which the [righteous] servants of Allah will drink; they will make it gush forth in force [and abundance].

• Verse 5 and 6 are describing the bliss one would gain in paradise.

Verse 7:

In Arabic (the exact wording):

يُوفُونَ بِالنَّذْرِ وَيَخَافُونَ يَوْمًا كَانَ شَرُّهُ مُسْتَطِيرًا

The translation in English:

They [are those who] fulfill [their] vows and fear a Day whose evil will be widespread.

• a Day whose evil will be widespread is the Day of Judgment known also as the Day of Resurrection.

All in all, the three verses together mean that because those people made vows and fulfilled them when Allah responded to their wishes, in addition to their fear of the Day of Resurrection, Allah reward them by letting them into Paradise to enjoy its bliss.

---

Source