Is it obligatory for someone who has backbitten a Muslim and wishes to repent to inform the one he backbit about what he said, or is it sufficient to seek his absolution without mentioning the details of the backbiting if he fears his anger and alienation?

The preponderant opinion is that it is not obligatory to inform the person who was slandered if they are unaware of it. Rather, it suffices for the slanderer to seek forgiveness for their sin and to seek forgiveness for their brother. This is what many scholars have indicated, among them Ibn Muflih and Sheikh Taqi al-Din ibn al-Salah. Hudhayfah (may Allah be pleased with him) and Abdullah ibn al-Mubarak indicated that the expiation for backbiting is to seek forgiveness for the person who was slandered. This is because informing them might lead to harming them twice or causing discord and aversion.