What are the necessary conditions for hiring an animal to operate a mill (raha)?

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Hiring an animal to operate a mill requires two conditions: knowledge of the stone (hajar), either by sight or by description that establishes knowledge, because the animal's effort varies based on the weight/difficulty. The work must also be estimated, either by time (e.g., one or two days) or by measure of the resulting product (e.g., one or two qafiz). Furthermore, the type of ground material must be specified if it varies, as some materials are easier to grind than others.