Baba Metzia 6:4-5

Baba Metzia 6:4

If a person hired a cow to plow in the hills but he ended up plowing in the valley and the plow blade breaks, he is exempt (because the damage is unrelated to the change in venue). In he hired it to plow in the valley but he plowed in the hills and the plow blade broke, he is liable. If he hired the cow to thresh pulse but he used it to thresh grain instead, he is exempt if the cow slips; if he hired it to thresh grain but he used it to thresh pulse, he is liable because pulse is slippery.

Baba Metzia 6:5

If a person hired a donkey to transport wheat but he used it to carry barley, he is liable for its injury (since the same weight is a larger volume). If he hired it to transport grain but he used it to carry straw, he is liable because the load is too unwieldy. If he hired it to transport a fixed volume of wheat and he used it to transport that same volume in barley, he is exempt (because, if anything, he made the burden easier) but if he increased the volume, he is liable. Sumchos says in the name of Rabbi Meir that the volume one must increase a load in order to be liable is a seah (about 3.5 gallons) for a camel and three kav (half a seah) for a donkey.