The Ushr–Kharaj Oscillation

Al-Baladhuri describes lands on the Euphrates that moved between the tithe and the land tax over several governorships, until al-Mahdi made them all alms-land. ¹

These are lands whose people accepted Islam on them when the Muslims entered, and lands that went out of the hands of their people to Muslims by gifts, and became tithe-land after having been land-tax land. ¹

• al-Hajjaj returned them to the land tax.
• Umar ibn Abd al-Aziz returned them to the alms.
• Umar ibn Hubayra returned them to the land tax.
• Hisham ibn Abd al-Malik returned some of them to the alms.
• al-Mahdi made them all alms. ¹

The specific dates and page numbers are not in the data and need an additional source.