Sayyiduna Jarir bin Abdullah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) sent him to some people of Khath'am. They sought to protect themselves by prostrating, but he killed them. So the Messenger of Allah (peace and blessings be upon him) paid half the blood money for them, then said: "I am free of every Muslim who lives with a polytheist."
His words "he paid blood money for them" show more clearly that he gave it voluntarily.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / 16472