Is specifying a known measure or weight essential for the validity of a forward sale?
Chapter on Guaranteed Salaf (Advance Payment) for a Specified Term
Al-Mughni
Book of Sales
Primary text
The specification of the measure or weight is obligatory, and there is no known disagreement on this requirement. The measure must be specified using known measures or known weights common among the general populace.
Supporting text
If the quantity is determined by a container of unknown capacity or a specific, unknown standard weight (sanjah), the sale is invalid. This is because such instruments may perish, making knowledge of the exact quantity impossible, which introduces uncertainty (gharar) that the contract does not require.