What is the third condition required for the validity of a forward sale (Salaf)?

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The third condition requires that the subject of the sale (Muslam Fih) must be known by a specified measure, whether by known volume (kail), known weight (wazn), or known count (adad). This requirement is based on the saying of the Prophet, peace be upon him: "Whoever enters into a Salaf must do so for a known measure, or a known weight, for a known term." This is obligatory because the subject of the sale is a non-present consideration established in the debtor's liability, thus necessitating the knowledge of its quantity, similar to the price.