Is it permissible to retract the relinquishment of a plot of land to a brother, after ten years have passed, given that the one to whom it was relinquished has not taken possession of the land and has not declared his acceptance of the gift?

What you have given as a gift to your brother, and his renunciation of it for the sake of Allah to build a mosque or a Quranic school, is considered his possession of it. If you are an adult of sound mind, you are not permitted to retract it, based on the Prophet's (peace be upon him) saying: "The one who retracts his gift is like a dog that returns to its vomit." Your brother is not allowed to dispose of it except for the benefit of the mosque or the school.