What is the ruling on differentiating between children by leaving a will with a sum of money for one of them to the exclusion of another, even after the testator's death?

Scholars have agreed upon the desirability of justice among children in giving gifts and the reprehensibility of favoritism. The majority have gone to the extent of deeming it desirable, while the Hanbalis have gone to the extent of deeming it obligatory and prohibiting favoritism except for a reason.

As for a will (wasiyyah), it is not permissible for an heir unless approved by the mature, sane heirs, based on the Prophet's (PBUH) saying: "Indeed, Allah has given every rightful person his right, so there is no will for an heir," and his saying: "There is no valid will for an heir unless the (other) heirs wish it." If parents make a will for some of their children, the will is not executed unless approved by all other heirs. The injustice of parents does not permit children to be disobedient (aqooq); rather, advice should be given gently, and kindness and obedience should be persistently shown in what is good.