Is it permissible to define the measure of the subject of sale by referencing a specific person's known measure or scale?
Chapter on Guaranteed Salaf (Advance Payment) for a Specified Term
Al-Mughni
Book of Sales
Primary text
If a specific person's measure or scale is stipulated, and both are generally known among the common people, the contract is valid. However, the contract is not restricted solely to those specific instruments. If those specific measures or scales are not generally known, the contract is invalid.
Supporting text
Scholars, including Al-Thawri, Al-Shafi'i, Abu Hanifa and his companions, and Abu Thawr, unanimously agreed that defining the measure of food by an uncalibrated measuring vessel (qafiz) or a garment by the measurement of a specific man (like saying 'by the measure of so-and-so') is impermissible, as the contract becomes void if the measuring standard perishes or the person dies.