What is the required specification for a slave stipulated as consideration in a valid writing contract?

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Those who validated the writing contract using a slave obligated that the slave stipulated must be a medium quality slave, specifically the Sindi (from Sindh), and considered medium in its category, similar to rulings concerning the Mahr (dowry). It is not permissible to stipulate an absolute, unspecified animal other than a slave, nor an unspecified garment or house. This prohibition extends to stipulating a specific garment from one's collection or a specific turban from one's collection, or other indeterminate items. However, if these items are described with the specifications required for a Salaf contract, the contract is valid.