1) IS AN "EVED" LIKE "KARKA"
QUESTION: The Gemara cites a Machlokes Amora'im concerning whether or not a lender of a loan with a Shibud may collect Avadim for his loan just as he may collect Karka. The Gemara explains that the Machlokes Amora'im depends on whether an Eved is like Karka or like Metaltelin. Other Halachos that are dependant on this question include whether a Pruzbul may take effect on an Eved, and whether Avadim may be acquired through Kinyan Agav with Karka.
Why do the Amora'im debate this point? The Mishnah later (96b) itself discusses this explicitly. In the Mishnah there, Rebbi Meir and the Chachamim argue whether or not Avadim are like Karka, and whether or not the Halachah of "v'Heshiv Es ha'Gezeilah" applies to stolen Avadim. (Rebbi Meir says that they are not like Karka.) Moreover, the Mishnah in numerous places teaches that all of the Halachos that are unique to Karka apply to Avadim because of a Hekesh which compares Avadim to Karka (for example, there is no Shevu'ah, Ona'ah, or Tashlumei Kefel for Avadim (Bava Metzia 56a)). (TOSFOS DH Ana)
ANSWER: TOSFOS explains that with regard to Halachos that are mid'Oraisa, the Chachamim (and Mishnah) who argue with Rebbi rule that Avadim are comparable to Karka. The Amora'im in the Gemara here are discussing Halachos instituted by the Rabanan. Perhaps with regard to Halachos d'Rabanan, Avadim are not like Karka, even according to the Chachamim who argue with Rebbi Meir.
The words of Tosfos are difficult to understand. There is a general principle that "Kol d'Sakun Rabanan k'Ein d'Oraisa Takun" -- the Rabanan made their enactments similar to the laws of the Torah. Accordingly, Avadim should be like Karka even with regard to enactments of the Rabanan. Moreover, according to Tosfos the Amora'im should argue whether a Shevu'ah d'Rabanan (such as Nishba'in v'Notlin, Shevuos 44b) may be made on Karka, and yet there is no such Machlokes.
It seems that Tosfos' intention is to differentiate between the Takanah d'Rabanan of Shibud Nechasim and Halachos that are mid'Oraisa because of the logic of the RASHBAM (Bava Basra 128a, DH Rav Nachman). The Rashbam explains that the reason Shibud takes effect only on Karka is that a person is not "Somech Da'as" -- he does not rely on collecting anything other than Karka, since movable items can easily be hidden from him. Therefore, the Rabanan instituted that a Shibud takes effect only on land. (See also TOSFOS to Yevamos 99a, DH Mani.)
The same logic clearly applies to a Pruzbul. The reason why a Pruzbul may be made only when the borrower owns land is that land is considered to be collected the moment Beis Din wants to collect it, since the owner cannot hide it from Beis Din's grasp (see Rashi DH Chal Al ha'Karka).
According to Rashi in Gitin (37a), the reason why the borrower must own Karka in order for the Pruzbul to take effect is that it is unusual for a lender to lend money if the borrower has no land, and the Rabanan did not institute the enactment of Pruzbul in unusual circumstances. According to this reasoning as well, the reason why a person normally lends money to someone who has land is that he knows that the land cannot be hidden from him and will be Meshubad to his loan.
The NIMUKEI YOSEF in the name of the RE'AH (34b of the pages of the Rif) adds that this is also the reason why the Chachamim instituted that Metaltelin are acquired through a Kinyan Agav with Karka. He explains that because land is immutable, a person always considers Metaltelin secondary to Karka. Therefore, when he makes a Kinyan on Karka, Metaltelin -- which are secondary to the land -- are automatically acquired together with the land.
Accordingly, Tosfos may intend to say that the reason why the Rabanan singled-out land from other possessions is different from the reason why the Torah singled-out land from other possessions. When the Torah gives a unique status or particular Halachah to land, it does so because the ownership a person can exercise over his land is not as strong as his ownership of Metaltelin: he cannot take it with him anywhere or do with it as he pleases. The same applies to Avadim which a person cannot change as he pleases, since the Eved is a living person. That is why, when the Torah singles-out land, it does so to teach that a certain Halachah does not apply to it as it applies to Metaltelin. In contrast, when the Rabanan single-out land to teach that a certain enactment applies only to land and not to Metaltelin, they do so because a person feels more confident ("Somech Da'as") with land than with Metaltelin. Hence, the rule that "Kol d'Sakun Rabanan..." obviously does not apply, since the Takanah d'Rabanan here is not k'Ein d'Oraisa but it follows an entirely different logic.
This approach answers the second question as well. If the Rabanan would enact a Shevu'ah, they certainly would say that the Shevu'ah cannot be made on land, k'Ein d'Oraisa, since the Torah's logic for singling-out land with regard to Shevu'ah applies even to a Shevu'ah d'Rabanan. Therefore, "Kol d'Sakun Rabanan k'Ein d'Oraisa Takun" applies and teaches that a Shevu'ah d'Rabanan cannot be made on land.
The Nimukei Yosef takes this approach further and asserts that even if Kinyan Agav is mid'Oraisa, it still may be true that the reason why Karka is unique with regard to this Halachah d'Oraisa is not the same reason why Karka is unique with regard to Shevu'ah and other Halachos. The reason is that a person is "Somech Da'as" more on Karka. Hence, the Gemara here might even follow the opinion that "Shibuda d'Oraisa" (which is the Halachic opinion, as the Rashba points out).
2) ACQUIRING "AVADIM" THROUGH ACQUIRING LAND
QUESTIONS: The Gemara cites a contradiction between two Beraisos concerning whether a person may acquire Avadim by making a Chazakah on Karka. The Gemara answers that -- whether Avadim are like Karka or like Metaltelin -- the contradiction between the Beraisos may be answered by explaining that one can acquire Avadim with Karka when the Avadim are standing in the land, and one cannot acquire Avadim with Karka when the Avadim are not standing there.
The Gemara asks what difference does it make whether or not the Eved is standing in the land? Even if the Eved is not standing in the land, he should be acquired through a Kinyan on the Karka. (If Avadim are like Metaltelin, then the Eved should be acquired through Kinyan Agav, and if they are like Karka, then the Eved should be acquired through Shmuel's Halachah that one can acquire ten fields through a Chazakah on a single field.)
The Gemara answers that the reason why the Avadim must stand in the field in order to be acquired with the field is that they move by themselves (and therefore they are not comparable to other Metaltelin or other Karka). (See Chart.)
(a) It is clear that neither Kinyan Agav nor the Kinyan of Shmuel applies in every case in which a person transfers Karka together with other Metaltelin or Karka. Kinyan Agav applies only when a person specifies that he wants the buyer to acquire the Metaltelin through Kinyan Agav with the Karka (Kidushin 27a). The Halachah of Shmuel applies only when the buyer has already paid for all of the fields that he wants to acquire, and not just for the field on which he performs the Chazakah (Kidushin 27b). Why, then, does the Gemara assume that the Beraisos are discussing a situation in which Kinyan Agav or the Halachah of Shmuel applies? Perhaps the Beraisos are discussing a situation in which those Halachos do not apply. The reason why the Eved must stand on the Karka is in order that he be acquired through Kinyan Chatzer; if he is in the Chatzer he can be acquired, and if he is not in the Chatzer he cannot be acquired. (TOSFOS DH Lamah Li)
(b) If the Gemara is not discussing Kinyan Chatzer, then what logic is there to differentiate between whether the Eved is standing in the land or not? If Kinyan Agav, or the Halachah of Shmuel, does not apply to Metaltelin or to movable Karka, then it should make no difference whether or not the Eved is standing in the land.
(a) The Rishonim offer different answers to the first question.
1. TOSFOS explains that an Eved normally is not prevented from fleeing when he is inside a field (that is, he is not "Mishtamer l'Da'as Ba'alo"), and therefore the Gemara assumes that the Beraisa is not discussing Kinyan Chatzer, since a normal Chatzer would not work to acquire an Eved in such a situation.
2. RABEINU YESHAYAH (in the Shitah Mekubetzes) explains that the Beraisa which says that an Eved can be acquired through a Chazakah on Karka is actually a quote from a longer Beraisa, and that longer Beraisa is identical to the Beraisa quoted earlier in all of the cases except in the case of Karka acquiring an Eved. Accordingly, the Beraisa taught that a person cannot acquire Metaltelin through an Eved. If the Beraisa would have been discussing Kinyan Chatzer in the case of Karka acquiring an Eved, it would not have said that Metaltelin cannot be acquired through an Eved since -- if the Eved is bound and the Metaltelin are resting on top of him -- the Kinyan of the Eved works for the Metaltelin on top of him as well.
3. With regard to the opinion that an Eved is considered like Karka, the question assumes that a piece of land can be Koneh other Karka (i.e. the Eved) through Kinyan Chatzer. Perhaps, however, this assumption is not correct, and an Eved cannot be acquired through Kinyan Chatzer if an Eved is considered like Karka.
Is there any basis in the Rishonim to prove that Karka can be acquired through Kinyan Chatzer?
There are several sources in the Rishonim for such a ruling. Tosfos and Rabeinu Yeshayah -- who find it necessary to explain how the Gemara knows that one does not acquire the Eved through Kinyan Chatzer (even though they are addressing the opinion that an Eved is like Karka) -- clearly maintain that one can acquire Karka through Kinyan Chatzer. (The OR SAME'ACH, Hilchos Gerushin 1:6, cites a source for this ruling from the Gemara in Bava Basra 87a, which says that one acquires what grows in the land through Kinyan Chatzer.)
The NIMUKEI YOSEF (Bava Basra 5a) rules that if a person builds a wall on his friend's land and he sells that wall to the owner of the land, the buyer acquires the wall through Kinyan Chatzer (even though the wall is attached, Mechubar, to the ground and is considered like Karka). Furthermore, the RASHBA in Gitin (22b) writes that the verse specifically teaches that one may not write a Get on Karka; without the verse a Get written on Karka would have been valid. However, the Rashba himself (77b) maintains that a Get may be delivered to the woman only through Kinyan Chatzer (or Kinyan Yad, placing it in the hand of the woman, where the hand is like her Chatzer; see Insights to Gitin there). The words of the Rashba imply that it is possible to transfer something that is attached to the ground (and has a status of Karka) to the woman through Kinyan Chatzer.
However, the NESIVOS HA'MISHPAT (241:3) cites proof from a Gemara in Gitin to the contrary. The Gemara in Gitin (22a) teaches that if one person owns a flower pot and another person owns the seeds inside of it, and he sells the seeds to the owner of the flower pot, the owner of the flower pot must make a Chazakah on the seeds to acquire them and he does not acquire them through Chatzer merely because the seeds are in his flower pot. If Kinyan Chatzer would work on Karka, the owner of the flower pot should acquire the seeds through Kinyan Chatzer.
The OR SAME'ACH answers that a flower pot cannot be considered a Chatzer which is supporting the seeds inside of it because what is inside the flower pot -- which has holes in it (it is an Atzitz Nakuv) -- receives its nourishment from the ground, and thus it is considered attached to the ground beneath the pot. Hence, the pot is merely like a towel wrapped around a plant which is attached to the ground.