ASALAM O ALAIKUM!! When I was immature and my parents feel that I am insecure so they took promise from me that I will give my cell phone to them every night. I fulfilled it till long time but now I have gained maturity and don't give it to them and they also don't ask for it. still I have to fulfill this promise or not. Is my imman incomplete by not fulfilling it?

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 is His slave and Messenger.

It is known that a young child who has not yet reached the age of puberty is not held accountable for his deeds, as the Prophet said: “The pen is lifted from three: an insane person until he regains reason, a child until he reaches puberty, and a sleeping person until he wakes up." [Abu Daawood, At-Tirmithi, An-Nasaa’i and Ibn Maajah] Ibn Taymiyyah said: “The scholars have unanimously agreed to its acceptance.”

Since your promise to your parents was made before you reached the age of puberty, then you are not sinful for not fulfilling the promise.

Even if your promise to your parents was made after you had reached puberty and they do not ask you now to give them the cell phone, then you are not sinful, Allaah willing, for not giving it to them; because if someone is promised something and is then content with the unfulfillment of the promise, then the one who made the promise is not sinful.

Allaah Knows best.