Reference for Bava Metzia 157:8

לא צריכא בשאין בדמיה ליקח אם יש בדמיה לשכור ישכור רב לטעמיה דאמר רב לא מכלינן קרנא

— Rab follows his view [expressed elsewhere], for Rab said: The principal must not be destroyed.<span class="x" onmousemove="('comment',' I.e., when an animal is hired for a certain task, e.g., to take a man on a journey, one cannot demand that the whole capital value of the animal shall be lost in order to fulfil the engagement. Hence, when the Mishnah states that if it died another must be provided in its place, it means that more money must be added to that realised by the carcase and another bought, so that the value of the carcase ultimately remains with the owner. But he is not bound to hire an animal for the money realised by the carcase for the completion of the task, the whole principal thus being lost to the owner. ');">⁸</span>

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