Shevuos Chapter 7, Mishnah 6. This mishnah too deals with cases of those who swear and collect - where a person takes an oath and on the strength of that oath collects - even though these cases were not listed in the first mishnah of the chapter.
The first case - the shopkeeper and the produce:
A person says to a shopkeeper: "Ten li b'dinar peiros" - give me a dinar's worth of produce, and the shopkeeper gives it to him. Afterward the shopkeeper says: "Ten li es hadinar" - give me the dinar, and the buyer responds: "Nesativ lecha unesato besoch pundascha" - I already paid you and you put the dinar into your purse. The question, then, is to whom the produce belongs: may the shopkeeper hold on to it, claiming that he has not yet received payment, or has the buyer, who claims that he paid, acquired it?
The Gemara explains that the case is one where the produce is not in the shopkeeper's possession and not in the buyer's possession either, but is lying in the public domain - so that neither of them holds it. However, since the shopkeeper admits that he sold the produce to the buyer, and the entire dispute is solely about whether the dinar was paid, the mishnah rules: "Yishava baal habayis" - the buyer takes an oath, and on the strength of that oath he takes the produce lying in the public domain.
The second case - the claim of "different produce":
Here too the produce is lying in the public domain, but in this case both sides agree that the buyer paid the dinar. The buyer says: "Ten li peiros" - give me the produce lying here, and the shopkeeper responds: "Nesatim lecha veholachtan lesoch beisecha" - you already took the produce you bought and brought it to your house, while the produce lying here is different produce and is not yours. In this case the ruling is: "Yishava hachenvani" - the shopkeeper takes an oath, and on the strength of that oath he takes the produce home.
The difference between the two cases lies in the question of admission: in the first case the shopkeeper admitted that he sold this very produce to the buyer, and the only doubt is whether he received payment for it. In the second case the shopkeeper claims that the produce lying in the public domain is not at all the produce he sold, and has no connection to the transaction. Since he does not admit this, he is regarded as the one in control of this produce - and even though it is not literally in his hand, his status is that of the one holding it, and therefore he swears and takes.
The opinion of Rabbi Yehudah:
Rabbi Yehudah disagrees regarding the last case and says: "Mi shehapeiros beyado - yado al ha'elyonah" - whoever has the produce in his hand has the upper hand. He does not mean the one who literally has the produce in his possession, but rather the buyer and not the shopkeeper: since the produce is not in the shopkeeper's domain but in the public domain, it is considered as though it is in the buyer's domain, and therefore it is he who swears and takes.
The parallel case - the money changer and the coins:
The mishnah brings a similar case, except that instead of a shopkeeper and produce it involves a money changer and coins (small change), with the coins lying in the public domain:
A person says to a money changer: "Ten li b'dinar maos" - give me a dinar's worth of small coins, and the money changer gives them to him. Afterward the money changer demands the dinar, and the buyer responds that he already gave him the dinar and that the money changer put it into his purse. Here the buyer takes an oath and takes the coins lying in the public domain, since the money changer admits that these are the coins he gave in exchange for the dinar, and the entire dispute is whether he received the dinar.
Both agree that the buyer paid the dinar, and he demands: "Ten li maos" - give me the coins lying here. The money changer responds: "Nesatim lecha veholachtan lesoch kisecha" - you already took your coins, while the coins lying here are different coins and are not yours. Since he does not admit that these coins belong to the buyer, the money changer takes an oath and takes them.
Here too Rabbi Yehudah disagrees and holds that the money changer does not swear and collect; rather, the customer who handed over the dinar is the one who acquires the coins, since "ein derech shulchani liten issar ad sheyitol dinaro" - it is not the way of a money changer to hand over the small coins before he has received the dinar. And since the money changer's claim is that he gave the coins first and only afterward received the dinar in exchange for them, this claim is implausible, and therefore it is the customer who swears and takes the coins lying in the public domain.
In summary: In this Mishnah we learned four cases of swearing and collecting with regard to produce and coins lying in the public domain. The principle that emerges from them: whenever the seller concedes that the merchandise before us is the buyer's purchase, and the dispute revolves around the payment alone, the buyer swears and collects; but whenever the seller claims that this is different produce or different money, he is not conceding any connection between the buyer and the item, and therefore he is considered the one in possession of it and he swears and collects. Rabbi Yehudah disagrees in the last two cases and holds that the buyer is the one who acquires, and in the case of the money changer he adds a reason drawn from the logic of common practice: it is not the way of a money changer to hand over an issar before he receives his dinar.