As we have learned previously, in a business transaction, it is essential that both parties make their intentions clear. What mechanisms does Jewish law have to clarify the intent of the buyer and the seller in the event that they disagree about the terms of sale?
Our Gemara brings a case where an ox was sold, and the purchaser discovers that he was sold a shor mu’ad. When he complains, the seller tells him that he sold him the animal for slaughter, so it should make no difference whether or not the animal is dangerous; the buyer claims that he bought it to work his fields and cannot use this animal.
In this case, Rav rules that the buyer can claim mekeh ta’ut – the entire transaction was made under a mistaken impression – and the sale can be cancelled. He bases this ruling on the fact that rov – the majority – of purchasers buy animals for field work, so the sale can be assumed to have been made with that in mind. Shmuel argues that following rov is only appropriate in cases of issur – discussions of what is ritually permitted or forbidden. In cases of mamon – money matters – we follow the principle of, “Ha-motzi me-havero alav ha-ra’ayah – the party that is making a claim on the other one must prove his claim.” Thus the purchaser would need to bring proof that he only bought the animal with the intention of using it in the field.
The Rashash asks why Rav cares about what most people do, given that our concern is the intention of this particular purchaser, and the Gemara has already established that he buys animals for both meat and for work. He explains that the case in the Gemara must be talking about a wholesaler who purchases both types of animals, but since most of his customers will want to buy animals for work, he prefers to buy those types of animals.