What is the authoritative standard for determining measure (kayl) and weight (wazn) for transactions?

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The authoritative standard for determining measure and weight is the customary practice (urf) in the Hijaz during the era of the Prophet, peace be upon him. This is the view held by Al-Shafi'i. The evidence for this is the narration from Abdullah ibn Umar, where the Prophet, peace be upon him, stated: "The measure (mikyāl) is the measure of Medina, and the balance (mīzān) is the balance of Mecca." Since the speech of the Prophet clarifies rulings, what was measured or weighed in the Hijaz during his time must adhere to that standard, and it is impermissible for it to change thereafter. If an item has no established standard in the Hijaz, one of two matters applies: either it reverts to the closest similar item established in the Hijaz (analogy, qiyās), or the local custom is considered, provided no legal standard exists. Wheat (barr) and barley (sha'īr) are established measures, supported by the Prophet's statement: "Wheat for wheat, measure for measure, and barley for barley, measure for measure." Similarly, other grains, spices (abāzīr), soapwort (ashnān), gypsum (juss), lime (nūrah), and similar items are measures. Dates are measures, as are all fruits subject to Zakat, such as raisins, pistachios, hazelnuts, jujubes, apricots, *batm*, olives, and almonds. Salt is a measure, based on the saying: "Salt for salt, measure for measure."