"and you shall return it to him": Consider how to return it to him. He may not feed "a calf to a calf, a foal to a foal" (i.e., he may not sell one of the animals in his keeping to feed the others.) And any animal which can work and eat (i.e., an animal whose work is worth the cost of its food), such as an ass or a cow, let it work and eat. "and you shall return it (also readable as "him") to him": He himself must be returned to him (if he is lost).
(Devarim 22:3) "And thus shall you do with his ass": It works and it eats.