Importance of keeping one's promise
Fulfillment of promise is a distinctive trait of the character of the prophets and the pious, and a normal code of conduct with all decent persons.
Breaking of a promise, on the other hand, is a habit of the wicked, mean and liars. A saying of the Holy Prophet (peace be upon him) is,
"A promise is like a debt." i.e. it is obligatory to honor a promise as it is to repay a debt. There is another saying of the Holy Prophet (peace be upon him) which means that,
"promise is an obligation for a believer."
Jurists are all agreed that a promise is a debt and its fulfillment obligatory, but only in the sense that its violation without a religious excuse is a sin though no remedy can be sought for its breach in a court of law.
In the language of the Jurists a debt is an obligation which is enforceable, while a promise is not.