[If] one rents a donkey on which to bring wheat, and he brings barley on it, he is liable; grain, and he brings straw on it, he is liable, because the added bulk is difficult to carry. [If he rents it] to bring a lesech of wheat, and he brings a lesech of barley, he is exempt; but if he added to its load, he is liable. How much must he add to its load to be liable? Sumchos says in the name of R’ Meir: A seah for a camel, three kavs for a donkey.