What knowledge is required when renting animals for threshing based on duration versus specific work?

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If the rental is based on a duration, knowledge of the animal's strength or weakness is necessary. If the work is not fixed by a duration, knowledge of the type (*jins*) of animal is required because the outcome differs; some animals have clean excrement while others have impure excrement. However, knowing the specific individual animal (*'ayn*) is not required.