1) WHERE "HA'MOTZI ME'CHAVEIRO ALAV HA'RE'AYAH" DOES NOT APPLY
QUESTION: The Beraisa discusses a situation in which a person gives a sum of money to a Shali'ach and instructs him to buy a certain item. The seller gives the Shali'ach a discount and sells him more of the item than that amount of money usually buys. Who profits from the seller's beneficence, the Shali'ach or the buyer who sent him? Rebbi Yosi rules that the answer depends on whether the purchased item is the type which always has a set price, or the type for which the price varies from item to item. If the item has a set price, the Shali'ach and the sender split the gain. If the item does not have a set price, the buyer who sent the Shali'ach keeps everything.
RASHI explains that the reason for this distinction is as follows. If the item has no set price, whatever the seller gives is considered to be worth the money given in return for it. If, on the other hand, the item has a set price, when the seller throws in extra items knowing that the money paid is not enough to cover the extra items at their set value, he clearly has intention to give them as a gift -- either as a gift to the Shali'ach or to the one who sent the Shali'ach. Since there is a doubt as to whom the gift was given, they split it.
Why should this uncertainty be grounds to split the extra items? Normally, in a case of doubtful ownership, the rule is "ha'Motzi me'Chaveiro Alav ha'Re'ayah," and thus the Shali'ach -- who is presently in possession of the extra items -- should keep them. (MITZPEH EISAN)
(a) Most Rishonim suggest a different explanation for why the gain is divided between the Shali'ach and sender. The extra merchandise is not considered a gift given intentionally to either the sender or the Shali'ach. Rather, it is considered either a mistake of the seller, or the seller simply decided to give them an unusually good deal. If this is the case, it is logical that the money should be split between the Shali'ach and the sender -- they both are responsible for the profit: the Shali'ach because he negotiated the deal with the seller, and the sender because it was his money with which the deal was made (see RIF cited by the ROSH 11:15 and others, based on the words of the Yerushalmi in Demai 6:9).
In a similar vein, the MORDECHAI cited by the HAGAHOS ASHIRI writes in the name of his father that the money is divided because the seller either made a mistake or decided to give them an unusually good deal. Accordingly, it was neither the sender's money that made the profit nor the Shali'ach's skills of negotiation, but rather just plain luck. The combined luck of the Shali'ach and the sender is assumed to have caused the gain, and therefore they split it.
However, this approach does not clarify Rashi's explanation.
(b) The principle of "ha'Motzi me'Chaveiro Alav ha'Re'ayah" is not an absolute rule. The Gemara in Bava Kama (46a) records a Machlokes between the Chachamim and Sumchus. In a case of monetary dispute, does the principle of "ha'Motzi me'Chaveiro Alav ha'Re'ayah" apply (Chachamim) or "Cholkin" (Sumchus), the money is divided between the two disputants? Although Tosfos in Bava Basra (35a, DH u'Mai) and other Rishonim rule that the Halachah does not follow Sumchus, some Rishonim do rule like Sumchus, at least in certain situations, because in many places the Gemara seems to follow the view of Sumchus and says that money in dispute is divided. This appears to be the opinion of the RASHBAM (Bava Basra 63a, DH Tenu) who rules like Sumchus ("Cholkin") at least in some cases, as Tosfos (ibid.) writes.
Accordingly, it could be that Rashi maintains that the Shali'ach and the sender divide the gain because of the ruling of Sumchus. (Since there is no clear Chazakah in this case due to the doubt whether the Shali'ach was given the object to hold for the sender or to hold for himself, the Halachah follows Sumchus in such a case.)