Is it permissible to substitute the intended cargo when renting a beast of burden?

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If a beast of burden is rented to carry iron, it is not permissible to load it with cotton, because cotton disperses and is affected by the wind, tiring the back. Conversely, if rented to carry cotton, it is not permissible to carry iron, because iron concentrates in one spot, making the load heavier, whereas cotton is distributed, lessening the harm.