Mannat Along With Dua
It is allowed, it is called nadhr (vow). God has ordered us to fulfil our vows if we make one. See verse 22: 29 ثُمَّ لۡیَقۡضُوۡا تَفَثَہُمۡ وَ لۡیُوۡفُوۡا نُذُوۡرَہُمۡ وَ لۡیَطَّوَّفُوۡا بِالۡبَیۡتِ الۡعَتِیۡقِ ﴿۲۹﴾ nadhr (vow) is also a form of an oath. So it will also be atoned in a similar manner in case it is broken: كَفَّارَةُ النَّذْرِ كَفَّارَةُ الْيَمِينِ The atonement of a nadhr is the same as that of an oath. The atonement is Allah will not hold you accountable for your inadvertent oaths, but He will definitely hold you accountable for oaths you swear with solemn intention. So if such an oath is broken, its atonement is the feeding of ten needy persons of a standard with which you normally feed your own families or the clothing of ten needy people or the liberation of one slave. But whosoever cannot afford these should fast for three days. This is the atonement for the oaths when you have sworn. And be true to that which you have sworn. Thus Allah explains to you His verses that you may be grateful. (5:89)