Correction and analysis: In the manuscripts the word was "unzur", and what we have established is taken from the source works. Its meaning is that he was among the close confidants of the liar, and by the liar is meant "al-Mukhtar bin Abi Ubayd al-Thaqafi".

Grading of the chain: Its chain is sahih.
Technical note: However, there is disagreement among those narrating from Abd al-Malik bin Umayr over the name of his shaykh: Qurrah and Shu'bah narrated from him that the shaykh's name is "Amir bin Shaddad", while most narrators narrated that the name is "Rifa'ah bin Shaddad", and this (the name Rifa'ah) is correct. Several narrators corroborated Abd al-Malik and gave the correct name "Rifa'ah".

Reference / Source: Al-Bazzar (2307) narrated it from Muhammad bin al-Muthanna, from Abu Amir al-Aqadi (and he paired Wahb bin Jarir with him), with this same chain.

Reference / Source: Al-Nasa'i (8688) narrated it via Khalid bin al-Harith, from Qurrah bin Khalid, from Abd al-Malik, from "Amir bin Shaddad".

Criticism and praise (jarh wa ta'dil): Imam al-Bazzar says: "Qurrah made a mistake in it, because he said 'from Amir bin Shaddad', whereas the correct version is what Abu Awanah said (i.e., Rifa'ah), and several narrators corroborated Abu Awanah."
Research note: We say: Qurrah bin Khalid is not alone in naming him "Amir bin Shaddad"; rather Shu'bah corroborated him, as mentioned by Abu Nu'aym in "Ma'rifat al-Sahabah" (4/2206) and by al-Mizzi in the biography of Rifa'ah in "al-Tahdhib" (9/206). Perhaps the mistake came from Abd al-Malik bin Umayr himself, for some scholars have mentioned that he deteriorated (at the end of his life) and had some errors.

Rejoinder: Nevertheless, a narration is also transmitted from Qurrah, through Abd al-Malik, with "Rifa'ah" (the correct name). Abu Dawud narrated it thus (continued)

Reference / Source: (continuation) al-Tayalisi (1382), and through him Abu Nu'aym in "Ma'rifat al-Sahabah" (5041) and al-Bayhaqi (9/142); and Uthman bin Umar bin Faris narrated it with Ibn Mardawayh in "Majalis min Amalihi" (34); both of them (al-Tayalisi and Uthman) narrate it from Qurrah (and give the correct name "Rifa'ah").

Reference / Source: Ahmad (36/21946, 21948 and 39/23701) and al-Nasa'i (8687) narrated it via Hammad bin Salamah, and Ibn Majah (2688) and al-Nasa'i (8686) via Abu Awanah al-Waddah al-Yashkuri; both of them from Abd al-Malik bin Umayr, from "Rifa'ah bin Shaddad", so both of them gave the correct name "Rifa'ah".

Reference / Source: Ahmad (36/21947 and 39/23702), Ibn Hibban (5982) and others narrated it via Isma'il al-Suddi, al-Tabarani in "al-Awsat" (7781) via Kathir bin Isma'il al-Nawwa', and in "al-Saghir" (38) via Bayan bin Bishr; all three of them narrated it from Rifa'ah bin Shaddad, with this same chain.
Text: Its wording is: "Any believer who grants a believer safety for his life and then kills him, I am free of the killer." Ibn Hibban and al-Tabarani, in his two narrations, added: "even if the slain man was a disbeliever."

Reference / Source: Ibn al-A'rabi narrated it in his "Mu'jam" (612), and through him al-Quda'i in "Musnad al-Shihab" (164).

Sources and chain: Via Mu'awiyah bin Salih, from Asim bin Rifa'ah, from Amr bin al-Hamiq, with the wording: "Faith has shackled al-fatk (treacherous killing); whoever grants a man safety for his life and then kills him, I am free of the killer, even if the slain man was a disbeliever." (In this chain) the narrator from Ibn al-Hamiq is named "Asim".
Technical note: Its chain includes "Rishdin bin Sa'd", who is weak. Imam al-Bukhari said in "al-Tarikh al-Kabir" (3/322): "Rishdin narrated from Mu'awiyah bin Salih, from Asim bin Rifa'ah al-Bajali, from Amr, from the Prophet (peace and blessings be upon him), and (the mention of) Asim in it is not sound."

Sources and details: Ahmad (45/27207), Ibn Majah (2689) and others narrated it via Abd Allah bin Maysarah Abu Layla, from Abu Ukkashah al-Hamdani, who said: Rifa'ah al-Bajali said: I entered upon al-Mukhtar bin Abi Ubayd in his palace, and I heard him say: "Jibril has only just now risen from my company." Rifa'ah said: I resolved to strike off his head, then I remembered a hadith that Sulayman bin Surad narrated to us from the Prophet (peace and blessings be upon him), that the Prophet (peace and blessings be upon him) used to say: "When a man entrusts you with his life, do not kill him." He said: Since he had entrusted me with his life, I disliked (killing him).
Technical note: The narrator made it part of the Musnad of Sulayman bin Surad, but its chain is weak, because Abd Allah bin Maysarah is weak and Abu Ukkashah al-Hamdani is majhul (unknown).