Their phrase “known” excludes an unidentified benefit; a contract for an unknown benefit is invalid.
Their words “from a specific asset or an asset defined by description, or for a known task” indicate that ijārah is of two kinds:
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A lease for the benefit derived from a specific asset or from an asset defined by description. An example of a specific asset is, “I lease you this house.” An example of an asset defined by description is, “I lease you a camel of such-and-such description for carrying or riding.”
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A lease for the performance of a defined task, such as transporting someone to a particular location or building a wall for him.
Their phrase “for a known period” means that the benefit must be leased for a fixed duration, such as a day or a month.
Their phrase “for known compensation” means that the amount to be paid must be stipulated.
It therefore becomes clear that the essential conditions for a valid ijārah, in either of its two forms, are: that the contract concern the benefit and not the asset itself; that the benefit be lawful and known; that, if the leased asset is not individually identified, it can be precisely defined by description; that the term of the lease be known; and that the compensation likewise be known.
A valid ijārah is sanctioned by the Quran, the Sunnah, and scholarly consensus. The Exalted says, “If they nurse (your children) for you, then give them their wages” (al-Ṭalāq 65:6), and He says, “Had you wished, you could have taken payment for it” (al-Kahf 18:77).