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 (definition) 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.