Whenever you make a promise fulfill it. Much emphasis has been laid on it in the Holy Quran and Sunnat. Allah, has ordered Fulfil your promise because you will be asked about it one the day judgement'.(Sura-e-Asr)
The Holy Prophet (s.a.w.a.) has said: Those who believe in Allah and hereafter, it is their duty to keep their promise.
(Usul-e-Kafi-2, p 364)
Hazrat Imam Sadiq (a.s.) has said: The promise of one believer to another believer is such a contract for which there is no Kaffara'. Therefore one who goes against his promise will be deemed to have disobeyed Allah and will incur his displeasure.
(Usul-e-Kafi-2, p. 363)
Keeping one's promise is considered by Allah as one of the qualities of Hazrat Ismail (a.s.). Believers have been strictly warned against breaking promises. Beware, don't say what you do not want to do. Allah does not like this at all.
Beware! Don't promise anything, which you are not in a position to fulfil. Breaking promise will bring disrepute.