Gemara

If Reuven sold 'my land and date trees', if he has date trees, he gives two. If he has no date trees, he buys two and gives them. If a creditor has a lien on the trees, he must redeem them (pay their value to the creditor to release the lien on them).

If Reuven sold 'my land with date trees', if it has date trees, he gives them (Bach's text - the sale is valid). If it has no date trees, the sale is void.

Rishonim

Rif and Rosh (4:12): Some learn from our Gemara that if one sells something that is easily bought, even if the seller does not have it, he is obligated to (buy and) give it. I disagree. If he does not have it, it is Davar she'Lo Ba l'Olam (and there is no Kinyan on it)! Rather, the seller has Reshus to buy them (to validate the sale). If the seller offers to buy them, the buyer cannot say 'I will take only a field with date trees.' Rather, the buyer must accept the land even though it itself has no date trees.

Hagahos ha'Bach: An old text of the Rif holds like the first opinion that he rejected. The Ra'avad (and Magid Mishneh below) had the old text of the Rif.

Rosh: Anything not in one's Reshus is Davar she'Lo Ba l'Olam, e.g. 'I sell to you what I will inherit from my father.'

Rambam (Hilchos Mechirah 24:13): If Reuven sold to Shimon 'my land and date trees', even if it had no date trees, if Reuven wants to buy two date trees, the sale is valid and Shimon cannot say 'I will take only a field with date trees.' However, if he sold 'my land with date trees', if it had no date trees, this is a Mekach Ta'os and Shimon can retract.

Ra'avad: The Rif obligates Reuven to buy two trees for Shimon.

Magid Mishneh: The Rif learns from our Gemara that if one sells something that is easily bought, even if the seller does not have it, he is obligated to (buy and) give it. We hold that one may contract (to supply Peros) based on a market price, even if the seller has none. A Tosefta teaches that if Reuven sold to Shimon with the understanding that Reuven has, and it is found that he does not have, he cannot uproot Shimon's rights. This is like the Ra'avad. Ba'al ha'Itur says in the name of Rabbeinu Nisim that Davar she'Lo Ba l'Olam applies only in a case such as dates that will grow on a tree, for both parties know and stipulate about Davar she'Lo Ba l'Olam. However, if the seller stipulates about something that is in the world, and the buyer gave money for it with this intent, the seller is obligated to fulfill his stipulation. The Tosefta (Bava Metzia 4:1) and Yerushalmi (Terumos 6:2) say so. Some explain like the Rambam, that the seller need not buy two trees; he may do so to validate the sale. Some ask, since the sale was Davar she'Lo Ba l'Olam, either can retract! Since it was not in the seller's Reshus at the time of the sale, it is as if it is not in the world. They also ask that this was two sales: a field and trees. Even if there are no trees, the field is sold. It does not depend on the sale of the trees! This is not necessarily so. What is the source that if one sold two things for one price, without setting a price for each, and he has only one, that the buyer must buy the one he has? He can say, I did not want one without the other! The Rashbam explains the Gemara to mean that he buys two trees lest he be unfaithful. His sale was a stipulation to buy two trees for the buyer, or return the money for two trees.

Gra (CM 216:8): Even though one may contract based on a market price, even if the seller has none, this is unlike Davar she'Lo Ba l'Olam.

Kesef Mishneh: The Rambam is like our text of the Rif. If we will say that since the trees were not in Reuven's Reshus, it is like Davar she'Lo Ba l'Olam, when Reuven buys two trees, why can't Shimon retract? If we will say that since date trees were in the world, it is as if they were in Reuven's Reshus (so it is Ba l'Olam), Reuven should be obligated to fulfill the contract! It seems that there are two aspects to this law: regarding the seller it is Lo Ba l'Olam, and regarding the buyer it is Ba l'Olam. This is because when the seller buys the trees, he fulfills his stipulation, even though at the time of the sale it was not in his Reshus, so the buyer must fulfill his obligation. If the seller does not buy them, he says that he did not own trees at the time of the sale and also now he does not own, so there was no Kinyan. We do not obligate him to buy to make it Ba l'Olam.

Taz (CM 216:5): We must say that date trees are available for sale. If not, it is not a sale at all (CM 211:1). We must say that they are not very available, for if so, Reuven would be obligated to buy and fulfill the sale.

Poskim

Shulchan Aruch (CM 216:5): If Reuven sold to Shimon 'land and date trees', even if Reuven had no date trees, if Reuven wants to buy two date trees, the sale is valid. Shimon cannot say 'I will take only land with date trees.' However, if he sold 'land with date trees', if it had two date trees, he acquired. If not, this is a Mekach Ta'os and he can retract.

SMA (14): This is like the Rambam. He holds that if there are date trees in the field, he need not buy others. Even though even without specifying, the sale included date trees in the field, he said 'and date trees' because we discuss when he did not show the land to Shimon at the time of the sale. This is unlike Ir Shushan, who says that even the Rambam obligates giving another two trees. However, if there are no date trees in the land, the buyer cannot say that he will retract because it is a Mekach Ta'os, for he expected to get land with date trees in it. Rather, as long as he did not say land with date trees, the seller may give land without date trees and two date trees from elsewhere. The Rambam holds that if he does not give two date trees from elsewhere, the buyer can totally retract. The Tur explains the Rambam like this.

Shach (1): The SMA argues with Ir Shushan because the Rambam did not mention giving another two date trees. I say that one cannot say that he acquired all the date trees. The Gemara says that if he has, he gives to him two! The Rashbam and Tosfos say that he gives only two, like the Gemara says, just the Rashbam says that he gives from elsewhere, and Tosfos says that he gives only from this field. Even though normally, he sold all the date trees, here he did not, since he mentioned (only two) date trees. The Ir Shushan is correct: The Rambam agrees with the Rashbam that he gives to him two date trees from elsewhere. Since he wrote that if he wants he may buy, we learn that if he has, he must give them. Further, this is obvious according to the Rambam. His primary Chidush is that if the seller buys from elsewhere, the buyer must fulfill the sale. However, the Rambam is unlike Tosfos. If he held that the extra words reduce the buyer's rights, he would have specified.

Rema: Some say that if he sold 'land and date trees', even if there are date trees in the field, if Reuven has other date trees elsewhere, he must give two. If he does not have, he need not buy. In any case Shimon gets the land, for these are two (independent) sales, of land and date trees.

Beis Yosef (DH Omarm citing the Rashbam): When he sold 'land and date trees', the Gemara says that if he has date trees elsewhere, he must give two, in addition to all date trees in the land sold. Some texts say 'If it (the land) has date trees, he gives two, even if it has many. This is wrong. Without specifying, the sale included date trees in the field. The extra words 'and date trees' give the buyer more rights. He should be no worse than when he did not say these words!

SMA (14): This opinion argues with the Rambam about two things. This opinion holds that there are two independent sales. The buyer gets the land as it is, with or without date trees. 'And date trees' is a separate sale of two date trees from elsewhere. If he has them, he must give them. If he does not have them, it is good for him to buy from elsewhere in order to fulfill his word, but we do not force him to do so. Since it is not in his Reshus, it is like a sale of Davar she'Lo Ba l'Olam, which does not acquire. The sale of the land remains; he deducts the value of two date trees.

Taz (CM 216:5): A case occurred in which Ploni sold two places in the Beis ha'Keneses, and it was found that he owned only one. The Rambam and Rashbam argue about this. We cannot force the buyer to pay unlike the Rambam's opinion; the sale is Batel. CM 182:8 is like this.