What is the basis for determining the minimum taxable amount (Nisab) for Zakat purposes?

Primary text

The Nisab is considered valid based on volume measurement (Kail), as the measure units (Awsuq) are volumetrically determined. Their transfer to weight (Wazn) was solely for standardization, preservation, and ease of transport. Consequently, the obligation of Zakat attaches to volumetric goods (Makilat) rather than weighted goods (Mawzunat). Ahmad specified that the Sa' (a volume measure) is equivalent to five ratls and one-third of a ratl of wheat. This standard was verified when the volume of lentils, being the most stable item for measuring, was measured, weighed, and found to equal five ratls and one-third, confirming this standard for the Prophet's measure established in Medina.