All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Al-Hattaab (from the Maaliki School of jurisprudence) stated: "On the other hand, a promise (Wa'd)] is not a binding commitment obliging the person to do – or not do – something now, but it is rather, as Ibn ‘Arafah defined it, ‘a statement that denotes the person's intention to do something good in the future’ …” [Tahreer Al-Kalaam fi Masaa’il Al-Eltizaam]
Al-‘Ayni said, “Conventionally, [a promise is to declare one's intention to do something good in the future.”
Hence, if there was an agreement to do a good deed in the future, it is considered a promise, and failing to do that good deed is considered a breach of the promise.
Please, refer to Fatwa 86010 about keeping promises.
Allaah Knows best.