Is it obligatory to return lands gifted to some children, to the rest of the heirs, or do they remain their property?

A father is obligated to be fair among his children in gifts and the division of inheritance, as Allah has commanded, based on the hadith of Nu'man ibn Bashir. As for maintenance (nafaqa), it is given according to the child's need. So, whoever needs money for studies or marriage should be given what suffices him, and the father is not obligated to give to one who does not need it. If a father gives his son a piece of land to spend from its proceeds on his siblings, there is no harm in that, provided it is for usufruct and not for ownership. Upon the father's death, his estate must be divided among all heirs, including the land he had registered in the name of one of his sons, lest the father incur the sin of injustice.