פירוש על בבא מציעא 6:5

והביא עליה שעורים – which are lighter than wheat

If one hired a donkey to carry wheat and he used it to carry [a like weight] of barley, he is liable [if the donkey was injured]. [If he hired it to carry] grain and he used it to carry [a like weight of] chopped straw he is liable, since the greater bulk is more difficult to carry. In the scenario mentioned in this section, although the person did not add weight to the load being carried by the donkey, he did add bulk. In such a manner he made the work more difficult and is therefore liable if the donkey was injured. In the examples given barley is bulkier than wheat and straw is bulkier than produce.

חייב – for its ruin/damage if he added three Kabs. And we don’t say that since the barley is lighter, he can add up to the weight of a Letekh of Wheat which is burden of the donkey (see Bava Metzia 80a).

[If he hired it to carry] a letech of wheat and it carried a letech of barley he is not liable. But if he increased the weight, he is liable. What increase in weight renders him liable? (1) Symmachos says in the name of Rabbi Meir: “One seah for a camel and three kavs for a donkey.” If he hired the donkey to carry a letech of wheat, which is a unit of volume, and he carried a letech of barley he is exempt if the donkey was injured. Although he changed the terms of the contract since he in essence lightened the load (barley is lighter than wheat) he is exempt. If, on the other hand, he actually increased the weight he will certainly be liable. The end of the mishnah then proceeds to ask how much weight increase will cause him to be liable. According to Symmachus (the name of a Sage) in the name of Rabbi Meir one seah per camel, which is 1/15 of a letech or three kavs (1/2 seah) for a donkey. Less than this and the hirer is not liable.

מפני שהנפה קשה – for the animal as a burden. For even though its burden is not as heaven as the burden of wheat, their volume is like the volume of wheat and the volume is like a burden.

Questions for Further Thought:
• In mishnah three we learned that a hirer who does not abide by the terms of the contract is automatically liable for any damage. However, in mishnayoth four and five there are circumstances where he is not automatically liable even though he did not abide by the terms of the contract. How are these mishnayoth different from the previous one?

לתך – one-half of a Kor, and Kor is 30 Seah..

סומכוס אומר וכו' – and the Halakha is according to Sumkho, but if he added less than this measure, he is exempt if the donkey became injured and he only gives the cost of the addition alone.