Gemara

49b (Rava): Devarim sheb'Lev (unspoken intentions) have no bearing in law.

50a: A man sold his property with intention to ascend to Eretz Yisrael. He went, but was unable to settle there.

(Rava): Anyone who goes intends to settle. Since he was unable, the sale is void.

A man sold his property with intent to ascend to Eretz Yisrael. He never went.

(Rav Ashi): He could have gone if he wanted. (The sale stands.)

61a (Mishnah - R. Meir): A Tanai (stipulation) is invalid if it is not (doubled) like the Tanai made with Benei Gad and Benei Reuven (if they will cross the Yarden to fight the Kena'ani nations, they will receive Eretz Gil'ad; if they do not cross, they will not receive it.)

R. Chanina ben Gamliel says, the verse does not teach this. There was a need to stipulate that if they do not cross, they will inherit (like other Shevatim) in Eretz Yisrael.

Question: R. Chanina refuted R. Meir!

Answer: R. Meir holds that it would have sufficed to write 'If they do not cross, they will inherit amidst you.' "In Eretz Cana'an" is extra, to teach that stipulations must be doubled.

R. Chanina holds that had it not written "in Eretz Cana'an", one might have thought that they will inherit amidst Bnei Yisrael in Gilad, but not in Eretz Cana'an.

R. Meir holds that "amidst you" connotes everywhere that you inherit.

Bava Basra 137b (Rava): If one said 'take this Esrog Al Menas that you return it to me', and the recipient took it for the Mitzvah, he was Yotzei only if he returned it afterwards.

Rava teaches that a gift Al Menas to return it is a valid gift.

Gitin 75a: Shmuel enacted that a Shechiv Mera (dangerously sick person) who gives a Get should say 'if I do not die (from this illness), the Get is invalid. If I die, it is valid.'

Rishonim

The Rif brings the Mishnah.

Rambam (Hilchos Ishus 6:1): Every Tanai, in Kidushin, divorce, sales or other monetary matters, needs four things...

Rambam (2): The Tanai must be Kaful... If any of the four things was omitted, the Tanai is Batel. It is as if there was no Tanai. She is Mekudeshes or divorced or the sale or gift takes effect immediately.

Rambam (14): Some late Ge'onim said that Tanai Kaful is needed only for Gitin and Kidushin, but not for Mamon (monetary matters). One should not rely on this. We learn about Tanai Kaful and the other three Mishpetei Tanayim (conditions for a proper Tanai) from Benei Gad. This was not divorce or Kidushin! The early Ge'onim said so (Tanai Kaful is needed also for Mamon). This is proper.

Rebuttal (Ra'avad): I already wrote in Sefer Mishpatim why we should follow the later Ge'onim.

Magid Mishneh: The Rif (Teshuvah 31) holds like the latter Ge'onim. He answers the question (that we learn Tanai Kaful from Benei Gad, i.e. Mamon) by saying that only R. Meir learns from there. Indeed, R. Meir requires Tanai Kaful also for Mamon. We hold like Chachamim. Letter of the law, we never require Tanai Kaful. Shmuel was stringent, lest a Beis Din hold like R. Meir, and permit even if a simple (not Kaful) Tanai was not fulfilled. In such a case, we give her the stringencies of being divorced (or Mekudeshes) and not divorced (or Mekudeshes). The Ramban and Rashba hold like this. In Sefer Kinyan (Hilchos Zechiyah 3:8) the Ra'avad said so, but he did not bring a proof.

Ran (21a DH ha'Hu): The Gemara (Kidushin 50a) connotes that if Devarim sheb'Lev were valid, the sale would be Batel, even without a Tanai Kaful. Some infer that a monetary Tanai need not be Kaful. It cannot be that he made a Tanai Kaful, for then Rav Ashi would not say 'since he could go, the sale is valid'. We do not force a person to fulfill his Tanai!

Rejection (Ran): This is no proof. Perhaps there was a Tanai Kaful. In Rava's case, the Tanai was made beforehand, but not at the time of the sale. In Rav Ashi's case, at the time of the sale he stipulated 'if I am able to go...and if I cannot, it is Batel.' Perhaps R. Meir holds that Giluy Da'as helps like a Tanai Kaful, but if one made a Tanai and did not double it, he shows that he wants the Ma'aseh to take effect even if the Tanai is not fulfilled. However, it is true that we do not require Tanai Kaful for money, and all the more so for Isurim. However, the Rambam requires Tanai Kaful.

Rosh (Bava Basra 8:48): Rava says that he was Yotzei only if he returned the Esrog because Rava does not disqualify a Tanai that contradicts the Ma'aseh. Even though the Tanai was not Kaful, the Rashbam (Bava Basra 137b DH v'Im) says that Tanai Kaful is needed only for Gitin and Kidushin, due to the severity of Arayos. In Mamon, Giluy Da'as suffices; we follow what we assume that the person was thinking. R. Shimshon says that the Yerushalmi says so. This is wrong. It is a leniency to say that Get needs Tanai Kaful, and without Kefel, the Get permits in any case! Thecorrect text of the Yerushalmi says that in Gitin and Kidushin, R. Meir is stringent to say that (perhaps) we infer the positive from the negative, so it is a Safek. In all monetary laws, R. Meir requires a Tanai Kaful and all Mishpetei Tanayim. The Gemara did not elaborate (about Esrog). Obviously, the Tanai was proper!

Rosh (Gitin 6:9): Shmuel enacted a text for a Get of a Shechiv Mera with a Tanai Kaful. We cannot say that this was a mere stringency, for this would lead to disaster (lest people assume that Tanai without Kefel is Batel and the Get is valid). However, some matters require only Giluy Da'as, and some matters do not require even Giluy Da'as.

Poskim

Shulchan Aruch (EH 38:2): Every Tanai needs four things. The Tanai must be Kaful.

Shulchan Aruch (4): Some say that we require the four conditions only to be stringent, but not if it leads to a leniency.

Taz (4): If a monetary Tanai lacked any of the four, we do not obligate one to pay (due to the Tanai), for we are lenient not to force the Muchzak to pay.

Shulchan Aruch (CM 207:3): If one sold his Chatzer or field and specified that he sells in order to go to a certain place, it is as if he sold on Tanai. Therefore, if he was unable to go, he returns the money and gets back his land.

Rema: This refers only to land. Giluy Da'as does not help for Metaltelim. He must make a proper Tanai.

Gra (10): This is like Tosfos, who says that even monetary Tanayim require a Tanai Kaful. It is unlike the Rashbam.