What is the ruling on substituting riding for carrying cargo when renting a beast of burden?

Primary text

If a beast of burden is rented for carrying cargo, it is not permissible to ride it, because the rider moves, thereby aiding the back through movement. If it is rented for riding, it is not permissible to use it for carrying cargo, because the rider sits in one place, intensely straining the back, whereas cargo is distributed on the sides.