Question: This is difficult, as a Shevuas Heses was only instituted in the time of Rav Nachman, as is apparent from the Gemara (see Tosfos 46a, DH "b'di'Rabbanan"). Why should he swear?

Answer#1: The case is where the customer does not give the storeowner anything right now. He says that he already paid before he received the fruit. (The Maharsha explains that the Rabbanan instituted this Shevuah before the institution of Shevuas Heses. This was because it was abnormal to pay before receiving the fruit. He therefore must swear to back up his abnormal claim.)

Answer#2: Alternatively, Rabeinu Tam explains, the first part of the Mishnah is a case where the storeowner measured the fruit into the vessels of the customer, and they are still in the domain of the storeowner. (The fact that they are in the vessel of the customer lends credence to his claim that he paid.) The second case is where the customer gave a Dinar in the store, and the storeowner had not yet taken it. The novelty of this law is that the people claiming that they already gave the money can keep what they claim is theirs (even though it is only partially in their domain, see Tosfos Ha'Rosh).

Explanation: The entire Mishnah is discussing cases where people swear and take, until the Mishnah's statement, "And these swear without a claim." The reason they can swear and take is because the item is already partially out of the possession of their opponents in the case.

Explanation(cont.): The reasoning of Rebbi Yehudah (who argues on this in the Mishnah) is that he holds that a storeowner will not measure out fruit unless he received a Dinar. This is why he holds that the customer can take the fruit without having to swear. Similarly, in the case where the customer gave a Dinar in the latter part of the Mishnah, Rebbi Yehudah says that the storeowner must give him fruit, as he will usually take the Dinar and measure out the fruit. This is because the Mishnah is referring to a storeowner who does not give on credit.

Explanation(cont.): According to this, the explanation of the Beraisa in the Gemara is as follows. Rebbi Yehudah says, in which case do the Chachamim argue with me? They argue in a case where the fruit is gathered and placed etc. However, if he already swung them over his shoulder, they should admit to me that one who tries to take from his friend must show proof.

Question: This is difficult. This "when (etc.)" is not like other uses of the word "when (etc.)" Usually, this word comes to explain the Tana Kama's statement, whether the person using it is coming to argue or explain. (Rabeinu Tam is saying it means, "When do the Chachamim argue on me?")

Question(cont.): However, according to Rashi's explanation it is like a regular "when (etc.)."

Implied Question: You should not ask that the Gemara in Eiruvin (81b) says that whenever Rebbi Yehudah says, "when (etc.)" it is to explain the words of the Chachamim. Yet in this case, he is coming to argue on the Chachamim. (Why shouldn't we ask that explaining that he is arguing is against this rule?)

Answer: This is because all of the Sefarim in Eiruvin (ibid.) have the text that this is when Rebbi Yehudah says this in a Mishnah. However, when he says this in a Beraisa, he indeed could be arguing on the Tana Kama.

Implied Question: Rebbi Yehudah did not say here that it is not the way of a storeowner to take fruit until he will take a Dinar, as he says in the case of the moneychanger. (Why didn't he use this argument here as well?)

Answer: This is because it is normal for a storeowner to give credit. However, in our case we are discussing a storeowner who does not normally give credit. (This is as opposed to a moneychanger, who never gives on credit.)

Implied Question: Later, as well, it is not possible to say that whoever has the Issar is considered to have the upper hand.

Answer: This is because the customer will sometimes give an Issar in exchange for Perutos (as opposed to a Dinar in exchange for Issarim). In such a case, one cannot say that whoever has the Issar etc. This is why the Tana writes a term that does not leave room for error.

Explanation: The Tana Kama holds that even so one only collects with an oath, as perhaps their father only said this in order that his children should look rich and that they should be held in high esteem.

Opinion: Rabeinu Tam rules that if someone lent money in order to receive half of the profit (of whatever business deal the borrower invests the money into), the borrower cannot be made to take an oath. Being that he takes money for his work, he will not make up a reason why taking more should be permitted. (See the Toras Chaim who says the Rambam argues on Rabeinu Tam.)