Musannaf Ibn Abi Shayba 28:1132

Ishaq ibn Mansur narrated to us, he said: I heard Amr ibn Abi Qays mentioning from Mutarrif, he said: We asked al-Hakam, I said: A king from the kings of Khurasan makes a peace treaty with the captives for a known head-tax (per capita). He said: Whatever is by way of a peace treaty, there is no harm in it.