Is it permissible for a father to allocate a portion of his wealth to one of his children without the knowledge of their siblings?

If a father singles out one of his children for a gift during his lifetime, this is not permissible, due to the obligation of justice among children, as the Prophet (peace be upon him) said: "Be mindful of Allah and be just among your children."

However, if he intends for it to be in the form of a will after his death, it is not valid, as the Prophet (peace be upon him) said: "Indeed, Allah has given every rightful person his right, so there is no will for an heir."