Gemara

If Reuven sold his house to Shimon, and said 'when I get the money, return it to me', this is forbidden (it was a loan; Shimon enjoyed free rental for Ribis). If Shimon said 'when you get the money, I will return it to you', this is permitted.

(Rava): In the Seifa, Shimon said 'if I want, I will return it.' Therefore, it is a sale, and it is permitted.

(Ameimar): Reuven sold land to Shimon without Achrayus (responsibility to compensate him if a creditor takes it), and saw that Shimon was sad, so he promised 'if it is taken from you, I will compensate you, these are Pitumei Mili (empty words of placation).

Question (Rav Ashi): Is this because the buyer must stipulate, and here the seller stipulated? Rava explained that the Seifa permits when Shimon said 'when you get the money, I will return it to you', for he means 'if I want to.' Had he not meant this, it would be binding, even though Reuven should stipulate!

Answer (Ameimar): Rava means that (since Reuven needed to stipulate, but Shimon did,) it is as if Shimon said 'if I want, I will return it.'

A sick man who feared lest he die wrote a Get to his wife. He was groaning out of regret.

His wife: Don't worry. If you recover, I am yours!

(Rav Zevid): These are Pitumei Mili.

Question (Rav Acha mi'Difti): If they were not Pitumei Mili, would we say that she made the Get conditional? Only he can do that!

Answer (Ravina): One might have thought that he gives it like she wants (and she wants it to be conditional). Rav Zevid teaches that this is not so.

Bava Basra 77a: If one wanted to give a gift to Ploni, and said 'acquire the field for him on condition that you write the document', he can retract the document and the gift (until Ploni receives the document).

Rishonim

The Rif brings the Gemara verbatim.

Rambam (Hilchos Mechirah 11:2): (If one was Makneh land or Metaltelim, and the giver or buyer stipulated Tana'im that can be fulfilled (the Kinyan is valid only if the Tenai was fulfilled). This is when he acquired through a valid Kinyan. If he did not acquire now and stipulated that he will acquire only if he fulfills the Tenai, even if he fulfilled it he did not acquire, for this is Asmachta.

Rambam (11): If Reuven sold a field to Shimon and Shimon said on his own 'when you will have money, give it to me and I will return the land to you', it is a valid Tenai.

Kesef Mishneh: The Rambam explains that Rav Ashi held that Rava answered that in the Seifa, Shimon said 'if I want, I will return it.' I.e. Reuven has no claim in Beis Din. It is not a Tenai at all. Had he not said 'if I want', it would be a proper Tenai. Ameimar answered that if after the sale was finished, Shimon said 'when you will bring money, I will return it', this is like a new sale after the first sale. It is permitted, just like when Shimon said 'if I want.' When the buyer stipulated after the sale finished and no Kinyan was done about his words, they are Pitumei Mili.

Rosh (5:27): Ameimar taught that Reuven's acceptance of Achrayus was Pitumei Mili when at the beginning they stipulated that there will be no Achrayus, and the sale was not yet finished. E.g. money was given, but the document was not written yet, in a place where where they acquire through a document. Since the buyer initially agreed to buy without Achrayus, even though the seller later promised Achrayus, since the buyer did not repeat the stipulation (to demand that the sake be with Achrayus), he did not consider this promise primary. It was merely to soothe his sadness. He thought 'if I repeat the Tenai, he will retract. It is better to be silent.' Therefore, they were Pitumei Mili. However, if the seller promised Achrayus from the beginning, it is not Pitumei Mili. It is a proper Tenai. Even though the seller said so, and the buyer should say so, surely he bought according to what the seller said. The same applies to the Get. He decided with his mouth and heart to divorce without a Tenai, and she calmed him with words.

Hagahos Ashri: The general rule is, whenever one must stipulate, and he did not accept the Tenai through a Kinyan, rather, through mere words, he did not acquire. We say that had he insisted to acquire with the Tenai, the other would have retracted from his Pitumei Mili.

Rosh (Gitin 6:9): R. Meir requires every Tenai to be doubled (if yes... and if no...) We need not teach that he holds that the wife's words have no effect. Rather, even according to Chachamim, her words are Pitumei Mili.

Rosh (Bava Basra 5:5): Some ask why he can retract the document and the gift. They should write the document and fulfill the gift against his will, like other Tana'im, e.g. on condition that you give to him 200 Zuz! They answer that here the Tenai hurts the giver, and they cannot write a document that hurts him without his consent. If he made Shluchim to write it, he can be Mevatel the Shlichus. I say that he can retract even from a Tenai that they give to him 200 Zuz, for it is Pitumei Mili. It benefits the recipient, so the recipient must stipulate. Therefore, the giver can be Mevatel the Tenai and the gift. We do not say that the Tenai is Batel and the action is fulfilled, for surely it is a Tenai to benefit the seller, and if he retracts from the Tenai, also the gift is Batel.

Poskim

Shulchan Aruch (YD 174:1): If Reuven sold a field to Shimon and said 'when I will get money, return the land to me', Shimon did not acquire, and all the Peros he eats are Ribis. If Shimon told him on his own 'when you will have money, I will return the land to you', it is not a Tenai and Shimon acquires immediately. If when Shimon said this. Reuven strengthened it, it is a Tenai. If Shimon stipulated, even amidst the sale, and Reuven was silent and did not strengthen it, it is not a Tenai. It is Pitumei Mili. The sale is valid, since Reuven, who should have stipulated, did not do so.

Shach (2): If he said so at the beginning of the sale, even if the buyer said so it is a valid Tenai.

Shulchan Aruch (CM 207:1): If one was Makneh land or Metaltelim, and the giver or buyer stipulated Tana'im that can be fulfilled (the Kinyan is valid only if the Tenai was fulfilled).

Rema: Some say that if the seller stipulated a Tenai that helps the buyer, it is not a Tenai. It is Pitumei Mili. This is when at first he sold Stam, and before the sale was finished the seller made a Tenai. However, if the seller said from the beginning 'I sell on the following Tenai', even if it favors the buyer, the buyer bought only on this condition. Similarly, if they stipulated and afterwards wrote a document, surely it was based on the initial Tenai.

Tur: If the seller made a Tenai that helps the buyer, it is invalid, like the one who bought without Achrayus, and afterwards the seller promised Achrayus. The Rosh says that this is only when at first he sold Stam, and before the sale was finished the seller made a Tenai.

Beis Yosef (DH u'Mah she'Chosav Kivan): Talmidei ha'Rashba say that we conclude that since the buyer needed to stipulate and the seller stipulated, it is Pitumei Mili. The Ra'avad says that the Rishonim (Teshuvas Rif 286) say that there is no difference whether the buyer or seller stipulated. Rather, 'seller' refers to any stipulation before the sale began; it is valid. 'Buyer' refers to any stipulation after the sale finished. The buyer already owns it, so the Tenai is void. The Ra'avad says that it depends on who stipulated; we discuss in the middle of the sale, e.g. after Chazakah but before money wasa given, or after money was given but before the document was written in a place where where they acquire through a document. Alternatively, even before Chazakah or money was given, if they initially agreed to sell without Achrayus, and now he promises Achrayus, the buyer should have strengthened his words, because the seller did not need this. Since the buyer did not strengthen his words, we say that they were Pitumei Mili. Had the buyer stipulated, it would be a proper Tenai. However, if the seller stipulated from the beginning, it is not Pitumei Mili. It is a proper Tenai. Even though the seller said so, and the buyer should say so, surely he bought according to what the seller said. When they already began the sale, or agreed to a sale, then we say that a later promise is Pitumei Mili. Tosfos (66a DH Pitumei) holds like the Ra'avad.

Beis Yosef (DH v'Chosvu): Talmidei ha'Rashba cite the Rashba to say that even in such a case, it is Pitumei Mili only if he does not say it like a Tenai, rather, like mere narrative. If he said 'I sell on this condition', it is a proper Tenai. It seems that the Rambam holds like the Rishonim's Perush, that it does not depend only on who stipulated, only on when they stipulated.