Is speaking about injustice against a person considered backbiting, and what is the expiation for breaking an oath sworn on the Mushaf, and what is the authenticity of the story of the arbitration between Ali and Mu'awiyah?

A Muslim is not blameworthy for seeking justice from one who has wronged him and requesting fairness. The expiation for swearing by the Book of Allah and breaking an oath is to feed ten needy persons, or clothe them, or free a slave. If one cannot find these, then it is to fast for three days. As for the story of Abu Musa and 'Amr ibn al-'As, which historians have mentioned with unauthentic narrations, its main source is an unreliable narrator, Abu Mikhnaf. The correct account is that Abu Musa and 'Amr agreed that 'Ali should remain the Caliph of the Muslims in Kufa and Mu'awiya should remain the governor of Sham.