My son recently learned the machlokes between Rav and Shmuel regarding “ain holchin b’mamon achar harov”. The scenario in the gemara (Baba Basra 93): Reuvain sells an ox to Shimon, who discovers that this particular ox has a propensity to gore. Shimon promptly demands a refund, claiming his ox is a “lemon” – he needs the ox to pull his plow, and a wild ox that gores is unusable. Reuvain counters that the sale is valid because Shimon can butcher the ox and eat it. Shimon insists that he, like most people, purchase oxen to plow, not to eat. Is the fact that “rov” of the world purchases oxen to plow and not to eat sufficient reason for Shimon to get his money back, or can Reuvain claim that since Shimon never specified that he was purchasing the ox only for plowing, he has no obligation to grant a refund? The halacha is "ain holchin b'mamon achar harov" - the behavior of the majority of the world is irrelevant, as Shimon should have specified that he wanted an ox to plow with.
In other areas of halacha we decide issues based on “rov”, so why are mamonos cases different? In fact, in other dinei mamonon cases we also use “rov”! The gemara (Kesubos 15b) tells us that a lost object found in a mostly pagan city may be kept because we follow rov and there is no obligation of hashavas aveidah (see Tosfos there which I find very difficult to understand). And to strengthen the question further: the rule of thumb is “ruba v’chazakah – ruba adif”, rov is stronger proof than a chazakah. Even if we view Reuvain as having a chazakah on the money in his possession, Shimon’s claim buttressed by a rov should be stronger.
The Ketzos (280:2) suggests two different theories to explain this halacha:
1) The Terumas haDeshen suggests a distinction between our case, where Reuvain is in possession of the money, and cases of issur v’heter where no money is involved, or the case of an ownerless lost object. Since Reuvain is in possession of the funds, the rule of “hamotzei m’chaveiro alav hara’aya” requires that Shimon bring definitive proof for his claim. There is a well known yeshivishe chakira as to whether rov is a birur or hanhaga, whether it serves as proof to resolve a doubt, or merely tilts the scale in favor of an approach but never resolves the underlying question. The Th”D comes down squarely on the side of understanding rov as a hanhaga, but not proof. If all Shimon has is a rov to bolster his claim, he fails to meet the threshold of proof required to force Reuvain to give a refund.
(As an aside, I think the Th”D implicitly contains another chiddush. Why is it that “hamotzi mei’chaveiro alav hara’aya”, that definitive proof is required to take money away from the one who possesses it? According to the Th”D, the requirement for proof imust be based on more than the fact that the current owner is a muchzak, as the Th"D never challenged the assumption of the question that "ruba v’chazaha – ruba adif”, a rov is better proof than chazakah, even chezkas mamon, possession. Just something to think about...)
2) The Ketzos disagrees with the Th"D and argues that “ain holchin b’mamon achar harov” is not because rov is a lesser form of proof, as the Th”D suggested, but rather because possession is a superior form of chazakah that trumps all other claims. “Ruba v’chazakah – ruba adif” applies only when dealing with regular chazakos that stem from the halacha of maintaining status quo. Chezkas mamom, however, is a categorically different type of chazakah, a super-chazakah that is not rooted in maintaining status quo, but is rooted in the assumption that “kol mah shetachas yad adam shelo”, possession being the strongest indicator of ownership.
There is actually a third approach to this issue in Shu”T R’ Akiva Eiger (mh"t 103:5) that you can read on your own time, but I just wanted to set the table with this to discuss some other stuff (assuming I ever get more time to write).