This is a reference to a story about one of Jaʿfar al-Ṣādiq's slaves, together with someone else, who killed a person. When permission for retaliation was sought from al-Ṣādiq by the governor of Medina, al-Ṣādiq at first remained silent because his view was that two people could not be put to death for the murder of one person, later withdrawing his protection of his slave after the victim's family convinced the governor to put pressure on him. Hossein Modarressi cites this reference to explain that al-Ṣādiq was keenly attuned to social sensitivities.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).