פירוש על בבא מציעא 4:2

נתן לו מעות ולא משך ממנו פירות יכול לחזור בו – [both] this one and that one. And it is an ordinance of the Sages, for according to the Written Torah, money does acquire, as we found concerning property dedicated to the Temple, as it is written (Leviticus 27:19): “[and he shall give the money] and it shall pass to him.” And what is the reason that they said that pulling/drawing towards one’s self acquires and not money? It is a decree lest the purchaser leave his acquisition in the seller’s house for long time and a fire spreads in the seller’s neighborhood and he would not be troubled to do what was required to save [what he sold]. Therefore, they placed them in his permission to retract from it (i.e., the sale) if he desired, for since if they increased in value while in his possession, it would increase [in value] and he would retract from the sale and the profit would be his. It is considered by them to be his and he would go to the trouble to save [it].

How is this so? If [the buyer] drew the produce away from [the seller] but did not give over the money, he cannot retract. If [the buyer] gave the money but did not draw the produce away from [the seller], he can retract. (1) But they said: “He that exacted punishment from the generation of the flood and the generation of the dispersion (at the time of the Tower of will exact punishment from one who does not keep by his word.
Rabbi Shimon says: “He that has the money has the upper hand.”
The examples given in section one demonstrate the rules learned in the previous mishnah. For instance if Shimon is selling produce to Reuven and Reuven takes the produce into his possession but Reuven does not pay the money, neither of them can retract. Even if for example the price should go way down, Reuven still owes Shimon the money that was agreed upon at the time of the sale. If, however, Reuven paid Shimon and Shimon did not give him the produce, either can retract the sale. If, for instance, the price of the produce should go up, Shimon can renegotiate the sale. However, the mishnah adds that although reneging is legal, God will eventually punish those who do not keep their word.
Rabbi Shimon disagrees with part of the opinion in section one. According to Rabbi Shimon the one who holds the money, i.e. the seller, can change his mind until the buyer draws the produce to him. The buyer may not, however, change his mind once he has paid the money. According to the previous opinion (section one) if the produce had not been given over to the buyer, either the buyer or seller could change their mind.

אבל אמרו מי שפרע וכו' – even though he is able to retract from [the sale]. We curse him in the Jewish court and say about him: He who punished the men of the generation of the Flood and from the generation which witnessed the separation of races (i.e., the Tower of Babel) and from the men of Sodom and Gomorrah and from the Egyptians who drowned in the sea, he will punish him who does not stand by his word (although the court cannot compel him – see Tosefta Bava Metzia 3:14; Talmud Bava Metzia 47b), and afterward he returns to him his money.

רבי שמעון אומר: כל שהכסף בידו ידו על העליונה – Rabbi Shimon is referring to the matter of the First Tanna/teacher [of our Mishnah] who said: “he gave him money, but [the other] did not draw from him towards himself the merchandise – may retract,” whether it is the seller or the purchaser, and Rabbi Shimon comes to say that sometimes the person who has the money in his hand, the seller, who received the money, has the upper hand, and it is in hand to uphold the sale if he wants, but the purchaser cannot retract in it, such as [for example], the attic of the purchaser was lent to the seller, for now if the seller wishes that the sale is upheld, the purchaser cannot retract, even though he has not pulled the object towards himself. And what is the reason why the Rabbis have stated that “pulling” acquires and not money? It is a decree lest the seller will say to the purchaser: “your wheat has burned in the attic;” here where it is the attic of the purchaser, if a fire broke out , he would trouble himself and bring it (i.e., the wheat). But the Halakha is not according to Rabbi Shimon, but rather, even though the attic of the purchaser has been lent to the seller, all the while that he has not drawn [the wheat] towards himself, he can retract, whether he is the purchaser or the seller.