We'll talk about an animal that was rented and overloaded by the person who rented it. The Mishnah says, “Somebody rented a donkey, and he agreed that he was going to carry a certain amount of wheat on the donkey.” But instead of putting wheat onto the donkey as its load, he put barley on it. Barley is lighter than wheat, so he added the same amount of weight of barley as he would have if he had put wheat, but the volume of the barley was much greater. It was a bigger load on the donkey’s back, even though it weighed exactly the same as what he would have put under the agreement with the wheat. Then let's say the donkey became damaged from carrying all this barley. Chayav, the renter would be obligated to pay for the damage to the animal if the weight of the barley equaled the weight of the wheat that he was supposed to put, because the volume—how much he had to put onto the donkey—caused the injury.
Same thing with tevu'ah (grain). Let's say he had an agreement that he was going to put grain onto the donkey, but he put straw instead. Straw is lighter than grain, so he added to that load even though the straw weighed the same as the grain. The amount he would have put would have had exactly the same weight, but he added volume. He added more here—straw—and therefore it was more difficult for the donkey to carry, even though it was exactly the same weight that it would have carried if it was grain. We say the renter would be obligated to pay for the damage if the donkey got injured, because in both of these situations, the renter has to pay. Even though he didn't add any weight to the load, the extra volume is difficult for the donkey, and therefore it probably caused the injury.
In the next case, we'll talk about what happens if he did not add to the weight or the volume of the load. He just deviated from the owner's instructions. We know that barley is lighter than wheat. Let's say he had an agreement that he was going to rent a donkey and put a letech, which is a certain unit of volume, of wheat onto it, but he deviated from the instructions and put a letech of se'orim (barley)—that same measurement of volume—onto it, and let's say the donkey got injured. So he put the same volume, but it was lighter, because barley is lighter than wheat. We say patur, the renter would be exempt from paying for the damage. Even though he did not follow the owner's instructions, he did not put more volume than he agreed upon, and he actually lightened the weight of the load being carried by the donkey, because barley is lighter than wheat. Therefore, he would not be at fault, and he is not responsible for the animal that got injured.
But if the renter, in his own mind, rationalizes and adds more volume to the load, he may say, “You know what? I can add more volume. I agreed to a certain weight. I agreed to a letech of chitim, a letech of wheat, and that weighs much more. So, even though I want to substitute se'orim in there, I can put more than a letech.” In that situation, chayav, he would be obligated to pay if the donkey got injured, because the donkey got injured, possibly from the extra volume, as we mentioned above.
Now we'll talk about when a renter loads the animal with the material he agreed to load it with, but adds to the amount of the load he agreed to put there. The renter loads an animal with an agreed-upon material. Let's say he says he's going to put wheat there, and he puts wheat on it, but he agreed to a certain amount that he was going to put. How much does he have to go over that amount to be obligated if the animal is injured? Sumchos says in the name of Rebbe Meir that if he agreed to carry a letech, he's liable if he adds a seah onto a camel. A seah is one fifteenth extra of the load. A seah is six kav (a unit of volume). So he's adding: he agreed to have ninety on there, and he added six more. That would be a problem; that would make him chayav, obligated, if the animal got injured here. He added one fifteenth to the load over what he agreed upon. That's how it works: a seah for a gamal, for a camel, and three kav on a donkey. So on a donkey, it would be even one thirtieth of the load. A donkey is weaker than a camel. Therefore, even adding that one thirtieth of the load onto its back could cause an injury, and therefore he would be obligated in that situation.