Reference for Bava Metzia 157:5

היכי דמי אי דשכיח לאגורי תרעומות מאי עבידתיה אי דלא שכיח לאגורי אגרא בעי למיתב ליה

— In truth, it means that another is not obtainable [here] for hiring, [yet he is bound to pay for half the journey,] because he [the owner] can say to him, 'Had you desired to go as far as this [where it died], would you not have had to pay its hire?' Now, what are the circumstances? If he simply promised him an ass, without specifying which, then surely he is bound to replace it;<span class="x" onmousemove="('comment',' As stated above. ');">⁵</span>

Jastrow