What is the legal ruling regarding a mother's action of saving a portion of her son's monthly payment to her, and then offering it back to him? Is it permissible for the son to take this money from her? And would this money be considered part of his inheritance after her death if she bequeathed it to him?

The fundamental principle is the obligation to be just among children in giving gifts, based on the hadith of Nu'man ibn Bashir: "Fear Allah and be just among your children," and because singling out some of them is a forbidden injustice. However, it is permissible for a mother to specify a portion of the money she used to spend on her son, because some jurists mentioned that among the justifications for singling out a child are their righteousness and piety, or the large size of their family, or their engagement in seeking knowledge, or if she withheld from some of her children due to their immorality or innovation. Singling out a child is also permissible if the rest of the children consent, as this removes the reason for prohibiting favoritism, which is the instigation of enmity and severance of kinship ties. However, if the mother bequeaths this money after her death, this is a bequest to an heir and is not valid unless the other heirs permit it, based on the Prophet's (peace and blessings be upon him) saying: "Indeed, Allah has given every rightful person their due, so there is no bequest to an heir."