SHAFAQNA – The Grand Ayatollah Shobairi Zanjani answered a question about breaking a promise.
Question: Is breaking a promise Haram?
The Grand Ayatollah Shobairi: Breaking a promise in dealings and contracts is Haram. Also in other cases if a person considers himself/herself responsible to fulfill a promise, according to precaution, it is not allowed to break it. But if the person does not consider himself/herself to be responsible and has just said that he/she will do such an act, it is Makroh to break it.
Source: PERSIAN SHAFAQNA