Gemara

44a (Mishnah): If Reuven paid for Shimon's Peros and did not yet do Meshichah, Shimon can retract. However, Chachamim said 'He who punished the generation of the flood... He will punish one who does not fulfill his word.'

R. Shimon says, whoever is holding the money has the upper hand.

47b (R. Yochanan): Mid'Oraisa, money acquires Metaltelim. Chachamim enacted that only Meshichah acquires, lest one sell something in his house, and then tell the buyer 'it burned in a fire through Ones. The seller will strive to save it only if it belongs to him (until Meshichah).

49b: A case occurred in which Reuven paid money for wine. He heard that a powerful official of the king was marrying off a child. (Surely, he would confiscate people's wine for this.) Reuven retracted.

Rav Chisda: Just like Meshichah was enacted for sellers (that they can retract until Meshichah is done), also for buyers.

47b - Question: According to R. Yochanan, what do Chachamim and R. Shimon argue about?

Answer: Chachamim hold like Rav Chisda (also buyers can retract). R. Shimon disagrees.

Chulin 83a (Mishnah): At four times in the year (when there is great demand for beef), one who paid for meat from an animal can force the butcher to slaughter it, even if he sold only a small amount. If the animal dies, the buyer loses what he paid (for he acquired part of the animal.) At other times, one cannot force the butcher to slaughter it, so if the animal dies, the butcher loses.

Question: In the Reisha, how did the buyer acquire without Meshichah?

Answer (R. Elazar): At these four times, Chachamim lifted their enactment, and allowed money to acquire (meat).

Bechoros 13b (Beraisa): If a Yisrael bought silver from a Nochri, and found idolatry among it, if he did Meshichah after giving the money, he must cast the idolatry to the Dead Sea (he acquired it, and may not benefit from idolatry)!

Question: If money acquires from Nochrim, why does the Beraisa mention Meshichah? The Yisrael acquired through money alone!

Answer: The case is, the Nochri agreed to conduct according to the law of Yisrael (only Meshichah acquires). The Beraisa teaches that even though he gave money, if he did not do Meshichah, he may return it.

Gitin 52a: If orphans gave money to buy Peros, and the price went down, they are no worse than a commoner (before Meshichah, he could retract).

If the price went up, we should apply Rav Chanilai's law (orphans are like Hekdesh, and they acquire through money).

Rejection (Rav Shisha brei d'Rav Idi): No, this would be bad for the orphans. If the sale were valid, and a fire erupted where the Peros are (in the seller's premises), the seller would not toil to save it, since it is the orphans' loss.

Rishonim

Rif: R. Yochanan taught that Chachamim enacted that Meshichah acquires, lest a seller be sloth to save the item from a fire. This teaches that if one gave money but did not do Meshichah, and Ones occurred, the seller loses. The buyer can say 'give to me my purchase, or return my money.'

Rebuttal (Ba'al ha'Ma'or): Since money acquires mid'Oraisa, the buyer can retract only as long as the item exists in the world. We put the item in the seller's Reshus so he will want to save it, for perhaps the price will rise, and he can retract and profit. The Gemara says that Meshichah was enacted primarily of sellers, and was extended also for buyers. The Ra'avad says that the enactment was primarily for buyers! If orphans gave money for Peros, they can retract if the price changed, but not if the Peros burned. Also, Reuven was able to retract before the official took his wine. After it was taken, he could not. If not, he should have waited until it was taken! One could reject the proof and say that Reuven happened to retract immediately. However, if so the Gemara did not need to bring the episode! In Chulin, we asked why the sale stands even though there was no Meshichah (even though once money was given, one cannot retract after Ones!) This is because there, the sale is still valid. The buyer bought meat; Neveilah meat has the same form as slaughtered meat. It is like a change of price. Had the animal been totally lost, it would have been clear why he cannot retract. I realize that some Ge'onim disagree.

Ra'avad (Kasuv Sham): Initially, I held like the Ba'al ha'Ma'or, but I retracted to agree with the Rif. To induce the seller to save the sale item, we enacted that it is in his Reshus until Meshichah. Perhaps Reuven feared that he would not get back what he paid for the wine after Ones. The Gemara in Chulin does not say that if it died, the buyer can retract. Rather, it says that the seller loses (immediately), for it is in the seller's Reshus until Meshichah. There is no proof from orphans buying Peros. Surely, the primary enactment was for buyers; the enactment also helps sellers, if the price rises. Since the Peros are in the seller's Reshus, it suffices to allow him to retract. This is R. Shimon's reason. Chachamim hold that even so, we equate them. I say that if the Peros were lost or burned, the buyer can retract without a Mi she'Pora. He can say 'give to me the Peros, and I will pay.' R. Shimon is more lenient and allows retracting when the price changed, all the more so (the law is more lenient) when Ones occurred, and there is no Mi she'Pora. Rav Chisda permitted to retract from the purchase of wine due to Ones, and did not mention Mi she'Pora.

Milchamos Hash-m: R. Shimon holds that the enactment was for the seller (to enable him to retract). The Gemara asked, according to R. Yochanan, why do Chachamim argue with R. Shimon, and answered that they made a uniform decree and allow also the buyer to retract. Now, all the more so the seller will strive to save it! It would be ludicrous to say that the primary enactment was only to allow the buyer to retract (since the Peros are not in his Reshus). Both Tana'im agree that either can retract if Ones occurs. R. Shimon holds that only the seller can retract due to change of price, since he suffers the loss if Ones occurred. Chachamim hold that even though the buyer can retract due to a change of price, it is in the seller's Reshus for Ones. If not, he would not toil to save it. He could profit through retracting if the price rises, but he is as likely to lose if the price drops and the buyer retracts! Reuven just happened to retract before the wine was taken. Some say that the Gemara brought it to teach that there is no Mi she'Pora. The Rif connotes like this. R. Chananel and Ge'onim say that he must accept a Mi she'Pora. The Gemara teaches that Rav Chisda rules like Chachamim, and explains the Stam Mishnah to teach that the buyer can retract.

Rosh (13,14): When Reuven retracted out of fear lest his wine be confiscated, the Gemara did not say that he must accept Mi she'Pora, like it said in the previous cases. Rav Chisda ruled that Meshichah was enacted also for buyers, so he can retract. Mi she'Pora is only for one who retracts due to change of price, not if he fears lest he lose everything. The rule is, the curse is only if they agreed and money was given and he retracts due to a change in price. If he retracts to avoid a loss, e.g. confiscation of his wine, there is no Mi she'Pora.

Rambam (Hilchos Mechirah 3:6): If one gave the money for a purchase and Ones occurred before he took it, and he said 'give to me my puchase, or return my money', even though witnesses testify that it was lost due to Ones and the seller was unable to save it and did not slacken, he must return the money, because Chachamim enacted Meshichah.

Tosfos (Bechoros 13b DH Hachi): When the Yisrael found idolatry among what he bought from a Nochri, if he gave money without Meshichah, he can retract. There is no Mi she'Pora, for the Nochri agreed to conduct according to the law of Yisrael; Mi she'Pora is not a law. Alternatively, R. Tam explains that Mi she'Pora does not apply when there is a total loss.

Poskim

Shulchan Aruch (CM 204:2): If one gave the money for a puchase but Ones occurred before he took it, and he said 'give to me my puchase, or return my money', even though witnesses testify that it was lost due to Ones and the seller was unable to save it and did not slacken, he must return the money. Mi she'Pora does not apply here.

Beis Yosef (DH v'Chosav ha'Ran): The Ran (47b DH Kach) says that the Ra'avad holds that if one retracts due to fear of Ones, he does not accept Mi she'Pora. The Ge'onim say so only if the Ones already occurred, for he does not receive anything. If he retracts before the Ones, he accepts Mi she'Pora. R. Chananel and Rav Hai Ga'on and Ra'avan say so. Rashi holds that one cannot retract if Ones occurred to Metaltelim. The other Poskim say that he can retract without a Mi she'Pora. Talmidei ha'Rashba say that Ri Albargiloni holds like the Ge'onim, and support them from the Gemara. Rav Chisda allowed retracting because a buyer can retract, not because it is in the owner's Reshus for Ones. The Mordechai says that if one retracts due to fear of Ones, nothing exempts him from Mi she'Pora. Perhaps this is even if the Ones already occurred, for the same reason. The Rambam did not bring these laws at all. This suggests that he obligates Mi she'Pora in every case, even if Ones occurred, if he wants back his money.

Bedek ha'Bayis: It is more reasonable to say that he holds like the Ge'onim, for in Hilchos Mechirah (3:6) he says that if Ones occurred, the buyer can demand his money back (and did not mention Mi she'Pora), like the Ge'onim said. We can explain the Rif similarly, for he wrote similarly.

SMA (4): The seller cannot say 'when Ones occurs, I will return your money', but you cannot retract until then. The buyer can say 'I do not want the toil of going to Din. Also, perhaps you will not be able to pay me.'

Gra (4): The Mishnah in Chulin connotes that there is no Mi she'Pora for retracting. This is why it asks 'the buyer did not do Meshichah'!

Shulchan Aruch (ibid): Some say that the same applies to one who retracts because he fears to lose the entire sale item.

SMA (5): Some say that the Shulchan Aruch discusses a total loss, to exclude a partial loss, e.g. he fears lest the wine sour.

Shach (3): The Bach rules like the latter opinion.

Gra (5): This opinion learns from the case of the official. The first opinion holds that he must accept Mi she'Pora, just the Gemara only asked about whether or not he can retract.