What is the difference between the permissibility of gifting to children during one's lifetime and the Prophet's (PBUH) refusal to witness a gift of "injustice" to one child without the others?

Scholars differ on the ruling concerning a father's gift to his children. Some believe that equality among them is obligatory, based on the hadith of Nu'man ibn Bashir, where the Prophet, peace and blessings be upon him, said: "Fear Allah and be just among your children." Others hold that it is recommended but not obligatory, and they validate a father's gift to some of his children without including others. The preponderant opinion is that equality among them is obligatory, unless there is a legitimate Islamic justification, such as the poverty or disability of some of them.