RABAH IS REFUTED
Answer: Really, he did not give it in front of witnesses. Reuven did not see his garment by Shimon, therefore Shimon is believed.
Question (Rav Nachman bar Yitzchak - Mishnah): A craftsman does not get a Chazakah.
Inference: Someone else gets a Chazakah!
Question: What is the case?
If witnesses saw that the object was deposited, why would someone else get a Chazakah?!
Answer: Rather, there aren't witnesses that saw the object deposited, and even so a craftsman does not get a Chazakah!
This refutes Rabah.
ONE WHO GOT BACK THE WRONG GARMENT
(Beraisa): If Reuven took back his garment from the craftsman, and saw that it is someone else's, Reuven may use it until the owner comes for it;
If he took back someone else's garment from a mourner's house or a drinking house, he may not use it.
Question: What is the difference between the cases?
Answer (Rav citing R. Chiya): A person often tells a craftsman to sell his garment. (Rashbam - we assume that Ploni did so, and the craftsman mistakenly sold Reuven's, and knowingly gave Ploni's garment to Reuven until he can get Reuven's garment back to him; Tosfos - he intended that Reuven keep Ploni's garment in place of his own.)
(R. Chiya brei d'Rav Nachman): This is only if the craftsman returned another's Keli, but not if the craftsman's wife or children did so;
Even when the craftsman returns it, this is only when he said '(take this) garment', but not if he said 'your garment.' (Then, surely he thinks that he is returning Reuven's.)
A SWINDLING CRAFTSMAN
(Abaye): This is a ruse of swindlers of Pumbedisa. Shimon claims his garment from Levi (a swindling craftsman), who denies that he ever received it.
Shimon: I have witnesses who saw it by you. It is mine! (The witnesses are unsure about this.)
Levi: That was another's garment.
Shimon: Show it, and we will see whose it is!
Levi: I refuse.
(Rava): Levi justifiably refuses! The Beraisa says that Shimon gets his garment only if witnesses saw it (and knew that it was Shimon's) with the craftsman.
(Rav Ashi): If Shimon is clever, he will induce Levi to show it. He will say (in private) 'you are holding it because I owe you. Let us have experts appraise it, you will get what I owe you, I will get what is mine (i.e. the excess)!
(Rav Acha brei d'Rav Avya): Levi can say 'I already had it appraised (it is not worth more than you owe me).'
CHAZAKAH OF A SHARECROPPER
(Mishnah): A sharecropper gets no Chazakah.
Question: What is the reason? Formerly, he only received a portion of the Peros. Now that he has taken all the Peros for three years, this shows that he owns the field!
Answer (R. Yochanan): The case is, his family has always been sharecroppers on the field. (It is common for such sharecroppers to eat all the Peros for some years, and then the owners eat all the Peros in other years.)
(Rav Nachman): If a sharecropper hired sharecroppers to work the field (by themselves for three years), he has a Chazakah;
If an owner saw someone else appoint sharecroppers over his field, he would speak up!
(R. Yochanan): If a sharecropper hired sharecroppers to help him work the field, he gets no Chazakah;
Perhaps the owner authorized him to do so.
FACTORS THAT MAKE ONE IMPARTIAL
Question (Rav Nachman bar Rav Chisda): Can a sharecropper testify about who owns the field (or is he considered partial)?
Answer (Rav Yosef): Shmuel said that he can.
Question (Beraisa): He cannot testify.
Answer: The Beraisa discusses when the fruit is still in the field (the sharecropper did not yet get his share, so he would gain from his testimony). Shmuel discusses when he already took his share.
(Beraisa): An Arev (guarantor) can testify that the borrower owns a certain land (e.g. someone else claims the land);
This is only if the borrower owns other land (sufficient to pay the debt, so in any case the Arev expect that he himself will not need to pay).
The lender can testify that the borrower owns a certain land;
This is only if the borrower owns other land.
If Reuven bought land from Levi, and then Shimon bought from Levi, Reuven can testify that Levi truly owned what he sold to Shimon, only if Levi has other land. (If not, Reuven gains from his testimony. Tosfos - if we will find out that Reuven bought stolen land, he will be able to get back what he paid from Shimon's land; Rashbam - his testimony causes that Levi's creditor will collect Shimon's land, and not Reuven's).