Is a father entitled to refrain from achieving financial equality between his children (his married daughter and his son) on the grounds that his daughter is married and has a husband who provides for her?

Equal treatment among children in gifts, care, and affection is obligatory, as evidenced by the hadith (definition) of Nu'man ibn Bashir: "Fear Allah and be just among your children." The fact that a daughter is married does not negate her right to justice nor does it justify preferring her brother, even if her maintenance is no longer obligatory upon him. The father should be reminded of the Prophet's saying: "Treat your children equally, for if I were to prefer anyone, I would prefer women." There is no basis for the daughter to compare herself to her father's wife, for her right to maintenance is obligatory upon him, and anything beyond that is a good deed.