Bava Metzia, Perek 6, Mishnah 4. This Mishnah also continues to deal with the question of the halachah regarding a renter who violates the conditions of a rental agreement that were agreed upon between him and the owner.
The first part of the Mishnah - plowing on a mountain and in a valley:
"Hasocher parah lacharosh bahar vecharash babikah" - A person rented a cow for plowing, and the rental included the plowing equipment along with the cow: the plow dragged behind it, and the "kankan" - the metal share that cuts the earth - and even two workers who accompany the cow and operate the equipment. The condition of the rental was that the plowing would be done on a mountain. Mountains are rough, rugged, and rocky, and therefore more likely to cause damage to the plow and its share, whereas a valley is softer and less rocky, causing less wear and tear on the plowing equipment.
"Lacharosh bahar vecharash babikah - im nishbar hakankan, patur" - The renter is exempt. For even though he stipulated to plow on a mountain, a place that is hard on the plowshare, in practice he plowed in the soft valley, and if the share broke there - he certainly should not be held liable.
"Babikah vecharash bahar - im nishbar hakankan, chayav" - Here the agreement was that the plowing would be done in a valley, and contrary to it, the renter went and plowed in a mountainous, rocky, and harder area, which is damaging to a plow. Since the share broke on the mountain, the renter is obligated to pay compensation, because he was not permitted to plow under such harsh conditions.
A side point that arises here: The workers operating the plow, if they were negligent in their work - they are responsible for the damage, and they must pay the owner of the plowing equipment directly.
The second part of the Mishnah - threshing legumes and grain:
Threshing is done in this way: The cow walks in a wide circle and drags a board behind it that is pulled over the legumes or the grain, thereby separating the edible kernel from the chaff and the outer shell.
The basis of the halachah here is that legumes are relatively moist at the time of threshing, and therefore slippery, as opposed to other types of grain which first dry in the sun prior to threshing, and are therefore dry and not slippery.
"Ladosh bekitniyos vedash bitevuah - patur" - The agreement was to use the cow for threshing slippery legumes, and in practice he threshed dry grain which is not slippery. If the cow slipped and was injured - the renter is exempt, for had he threshed the slippery legumes as stipulated, it would have been even more prone to slip.
"Ladosh bitevuah vedash bekitnis - chayav, mipnei shehakitnis machalekes" - The agreement was to thresh dry grain which is not slippery, and the renter violated the agreement and threshed legumes. If the cow slipped, was injured, or died - the renter is obligated to pay for the damage, because legumes are slippery.
In summary: The Mishnah teaches via two pairs of cases - plowing on a mountain and in a valley, and threshing legumes and grain - that one who violates the conditions of a rental and uses an animal or tools under harsher and more dangerous conditions than what was agreed upon, is liable for the damage caused; whereas one who violates the condition toward the easier and safer direction - is exempt.