The Criteria of Promises
According to the prophetic hadith, one of the signs of a hypocrite is that “when he makes a promise, he breaks it.” [Muslim] Imam Khalil Nahlawi explains that what is meant by this hadith is that it is unlawful for one to make a promise while intending to break it, as such an intention is deemed hypocrisy.
If one intends to fulfill it, then making a promise is permissible.
One should strive his utmost to fulfill a promise, as doing so is sunna. Breaking a promise that one intended to fulfill, then, is disliked and unbecoming of a believer. (And if the promise were depended upon by another to make a financial commitment or significant undertaking, then it would be religiously binding to fulfill, as contemporary scholars clarify.)
[Nahlawi, Durar Mubaha fil Hazr wal Ibaha]
The above criteria would apply as well when making an internal promise to Allah Most High, without verbally pronouncing a vow or oath, as breaking the latter would necessitate expiation [depending on the vow/oath].