Gemara

48a (Beraisa - R. Shimon): Chachamim said that a Dinar does not acquire a garment, and we do not force the seller to give it, but 'He who punished... He will punish one who does not honor his word. If one verbally agreed to buy or sell, this is not a Kinyan. Chachamim are displeased with one who retracts.

(Rava): There is no curse for this, just Chachamim are displeased.

If one verbally agreed to buy or sell, and money was given, and he retracts, he is cursed. If one verbally agreed to buy or sell, but money was not given, and he retracts, he is not cursed.

A case occurred in which people paid R. Chiya bar Yosef money to buy salt. Salt rose in price. R. Yochanan told him 'honor the sale. If not, you will accept on yourself 'He who punished...''

They had given partial payment. R. Chiya thought that he must sell only as much as was paid. R. Yochanan told him that he must honor the entire sale.

49a: Rav Kahana received partial payment for flax; the price went up.

Rav: You must give flax for the money you received. The agreement to sell the rest is mere words. One who retracts from an oral agreement is not mi'Katnei Emunah (of small faith);

(R. Yochanan): He is considered mi'Katnei Emunah.

Question (Beraisa - R. Yosi b'Rebbi Yehudah): It says "Hin Tzedek" to teach that 'Hen' and 'Lav' (when you say yes or no) must be just.

Answer (Abaye): R. Yosi forbids saying something that he does not intend to fulfill. (However, one who intended to fulfill his word need not do so if circumstances (e.g. prices) change.)

Contradiction (Rabah bar bar Chanah citing R. Yochanan): If Reuven promised to give a gift to Levi, he is fully permitted to retract.

Answer (Rav Papa): This refers to a large gift (Levi is not confident of receiving it. He) forbids retracting from a promise for a small gift.

74b: (Rav Papa): R. Shimon said only that the buyer cannot retract if the price is the same. If it changed, also he may retract.

Support: If we would not say so, we do not find that a buyer ever bears the curse 'He who punished...' according to R. Shimonl

(Beraisa - R. Shimon): In any case, that is the law. However, Chachamim said, 'He who punished...'

'In any case' teaches that R. Shimon said that the buyer cannot retract only if the price is the same.

Kidushin 8b: Rav Huna's children agreed to buy a slave for Perutos (small coins). They had no Perutos with them. They left a piece of precious metal for a security. The value of the slave increased.

R. Ami: The coins were never given, so the security has no effect.

Rishonim

Rif and Rosh (4:10,14): If one verbally agreed to buy or sell, this is not a Kinyan. Chachamim are displeased with one who retracts. The curse is only if they agreed and money was given. The case of Rav Huna's sons proves that there is no curse even if a security was given.

Ba'al ha'Ma'or: If the price changed, one who retracts is not mi'Katnei Emunah. We find that R. Shimon says that money acquires for a buyer, but if the price changed, he can retract, but he is cursed (74b). The case of Rav Huna bar Avin's sons proves that if the price changed, one who retracts is not mi'Katnei Emunah. The Gemara brings Rav Kahana's sale of flax, and then the argument of Rav and R. Yochanan. In the case of the flax even R. Yochanan would have permitted him, for the price rose. It put these matters together because they are similar. The Rif rules like R. Yochanan. We challenge Rav from the Beraisa of "Hin Tzedek" Why did he bring Abaye's answer, which is unlike R. Yochanan? Perhaps it is to teach about a large gift; R. Yochanan.admits that one may retract from it.

Ra'avad (Kasuv Sham): R. Yosi learns from "Hin Tzedek" that one who promises falsely transgresses an Aseh. He is called a Rasha. This is worse than mi'Katnei Emunah! There was no question against R. Yochanan. He could hold like R. Yosi; he discussed mi'Katnei Emunah, for he came to argue with Rav. One who intended when he promised is not a Rasha. If he promised a small gift and does not give it, R. Yochanan holds that he is mi'Katnei Emunah.

Milchamos Hash-m: The Ba'al ha'Ma'or understands that 'two Tar'ei' mentioned in the Gemara means that the price went up or down. This is a total mistaken. R. Shimon holds that mid'Oraisa, money acquires. Chachamim did not enact that a buyer can retract. What difference does it make if the price changed? The Gemara said that one who contracts intends for the lower price. If so, according to Chachamim there is no Mi she'Pora (for one who contracts)! Rather, 'two Tar'ei' is when he gives money now on condition that not to receive Peros until later. The price is prone to change, and he gives money now with intent for the lower price later, so it is forbidden. Rav Huna's sons wanted to buy the slave. The seller wanted to retract because the price rose. R. Yochanan told them that the seller can retract; indeed, one who does is is mi'Katnei Emunah. The question from the Beraisa of "Hin Tzedek" was difficult also for R. Yochanan, who permits retracting from a large gift, because the verse does not distinguish large from small gifts.

Rosh (14): The general rule is, the curse is only if they agreed and money was given and he retracts because the market price changed. If they agreed without money, one who retracts is mi'Katnei Emunah, like R. Yochanan says.

Beis Yosef (CM 204 DH v'Chosav Ba'al): R. Ami would not have permitted Rav Huna's sons to retract if this is mi'Katnei Emunah.

Beis Yosef (ibid.): Talmidei ha'Rashba bring a proof from R. Chiya bar Yosef, who thought that he need not sell more than what he was paid for. He was not concerned for being mi'Katnei Emunah. However, perhaps he held that since partial payment does not acquire the full amount (for Mi she'Pora), there is no obligation to honor his word about the rest, either. It seems that Rashi holds that Rav and R. Yochanan argue even in a case when the price changed. He explains that Abaye answered for Rav that the Beraisa of "Hin Tzedek" forbids only saying something without intent to fulfill it, but if prices change, he may retract. This implies that R. Yochanan forbids even if prices changed! The simple Perush is that as long as he intended to fulfill it when he said it, he may change his mind later even if prices did not change. The Rashba holds that 'mi'Katnei Emunah' applies even if the price changed; the Ra'avad, Magid Mishneh, Ran and Tosfos agree.

Rif: The Halachah follows Rava, that Beis Din gives the curse. We learn that one who retracts is not considered one who acts like your nation.

Rambam (Hilchos Mechirah (7:2): Beis Din curses him and says 'He who punished... He will punish one who does not fulfill his word.

Rambam (8): If one agreed to buy or sell through mere words, it is proper to fulfill his word even if he did not receive any money, mark (a barrel) or take a security. If the buyer or seller retracts, even though he need not accept Mi she'Pora, he is mi'Katnei Emunah, and Chachamim are displeased with him.

Poskim

Shulchan Aruch (CM 204:1): If one gave the money but did not do Meshichah, even though he did not acquire the Metaltelim, if the buyer or seller retracts, he acted unlike a Yisrael, and he must accept Mi she'Pora.

Shulchan Aruch (7): If one agreed to buy or sell through mere words, it is proper that he fulfill his word even though he did not receive any money, mark (a barrel) or take a security. If the buyer or seller retracts, even though he need not accept Mi she'Pora, he is mi'Katnei Emunah, and Chachamim are displeased with him.

SMA (12): The Rambam holds that this is even if the price changed.

Note: He infers this, for the Rambam did not specify.

Rema (11): If the price changed, 'mi'Katnei Emunah' does not apply. Some forbid retracting even if the price changed. This is primary

Bach (Sof ha'Siman): The YerushalmI say that there is no problem of mi'Katnei Emunah when the price changed. It is best to say that the Bavli does not argue.