What is the ruling on the type of saddle used when renting for riding?

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If an animal is rented to be ridden bareback (ariyyan), it is not permissible to ride it with a saddle (sarj), as this constitutes using it for more than what was agreed upon. If it is rented to be ridden with a saddle, it is not permissible to ride it bareback, because riding without a saddle can cause friction (hami), potentially injuring the animal's back (aqraha). Furthermore, if rented to be ridden with a saddle, it is not permissible to use a heavier or more harmful saddle. For example, if a donkey is rented with its saddle, it is not permissible to ride it with a saddle belonging to a heavier steed (bardhun) if that saddle is heavier than the donkey's original saddle. If a beast is rented with a saddle, and ridden with a harness (ikaf) that is heavier or more harmful, it is not permissible; however, if the substitute is lighter and less harmful, it is permissible.