This, then, is what transpired in the land of the Sawād. The legal ruling requires that its kharaj remain the original assessment imposed upon it. If a new circumstance arises that, on the basis of the imams’ independent legal reasoning, necessitates changing it to crop-sharing, that change remains in force as long as its cause persists. Once the cause disappears, it is restored to its original state, for the imam may not overturn the independent legal reasoning of his predecessor.
As for making tax officials liable for the revenues of kharaj and ʿushr (tithe), this is invalid and has no legal standing in the Sharīʿah. The official is entrusted to collect what is due and remit what he receives. He is therefore like an agent who, once he has discharged the trust, is not liable for any shortfall and has no right to any surplus. Guaranteeing the revenues at a specified amount entails that the official retain whatever exceeds that amount while making good whatever falls short. This is incompatible with the nature of his office and with the legal requirements of a trust; therefore, it is invalid.
Aḥmad drew attention to this meaning in the narration of Abū Ṭālib, concerning one who takes on the ājām (thickets or wooded tracts) without knowing what they contain, or a tasūj (land district) without knowing what crops it contains. He said: “This is as bad as it can be.”
He likewise said, according to the narration of Ḥarb, when he was asked to explain Ibn ʿUmar’s hadith, “Tax-farming contracts are ribā (prohibited usurious gain).” He said: “It is when someone takes on a village containing non-Arab cultivators and date palms.” The wording of the hadith was narrated by Sufyān, from al-Aʿmash, from ʿAbd al-Raḥmān ibn Abī al-Zinād, from Ibn ʿUmar: “Tax farming is ribā.” Thus, it was called ribā. The meaning is that it has the same legal ruling as ribā with regard to invalidity and the corruption of the contract.
It is also related from Ibn ʿAbbās that he said: “Beware of ribā, and beware lest you place upon yourselves the shackles that Allah placed around their necks. Indeed, those are tax-farming contracts, and they constitute humiliation and disgrace.”