Upvote:5
This question has been here for a while, and the Wikipedia entry has slightly changed in its wording though not the context. In both cases I would suggest that the emphasis on taxation lies on the Persians and Romans and not on any nomadic tribesmen. This is further emphasised by the primary source of taxation being the land tax (and, therefore, the settled populations):
Changes in religion and the ruling classes did not modify the nature of these Middle Eastern Empires, which were typified by centralised governments ruled by elites who lived on the surplus extracted from the land. Taxation on the land was the main source of revenue for the State centralised.
—Campopiano, 'State, Land Tax and Agriculture in Iraq'
However, the question on whether Muslims agreements on taxation happened before or after the conquest does have a concrete answer:
According to juridical sources, Kharādj, a tax imposed on the land of conquered populations who had not accepted Islam before the conquest and had not signed a special agreement (ṣulḥ), was levied on most of the conquered areas.
—Campopiano, 'State, Land Tax and Agriculture in Iraq'
The above has a footnote directing to further sources:
For a general survey of these topics, see: D. C. Dennet, Conversion and the Poll Tax in Early Islam (Cambridge, Mass., 1950); F. Løkkegaard, Islamic taxation in the classic period with special reference to circumstances in Iraq, second edition (Philadelphia, 1978); H. M. Tabūtabā’ī, Kharāj in Islamic law (London, 1983), A. Oran, S. Rashid, “Fiscal Policy in Early Islam“, Public Finance 44, 1989, 75-101. See also : V. Lagardère, “Structures étatiques et communautés rurales: les impositions légales et illégales en al-Andalus et au Maghreb (XIe-XVe)”, Studia Islamica 80, 1994, 57-95.
I recommend the full text of the article as it is based on both Sassanian and Arab tax ledgers in an area of modern Iraq, and tries to assess why tax revenues declined from the conquest to the 10th century.