Majma al-Zawaid — Hadith 10940

Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said about this saying of Allah Almighty: Allah Almighty says: "If that person is from a people hostile to you and he is a believer, then (the penalty is) freeing a believing slave." Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: Previously a man would come to the Prophet (peace and blessings be upon him) and accept Islam, then he would go back to his people, who were polytheists, and during a military expedition or a battle the Muslims would kill this man too; so the one who had killed him would free a slave. Allah Almighty says: "And if he is from a people between whom and you there is a treaty." Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: This refers to the man who is a dhimmi and whose people have a covenant; the blood money will be paid to those people, and the one who killed him will free a slave.
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains Ata' bin al-Sa'ib, who had become mukhtalit (confused in memory).