Gemara

(Rav): If Reuven planted trees on Shimon's field without permission, he has the lower hand (he gets the lesser of the improvement or expenditures);

(Shmuel): We estimate how much people pay to plant such a field.

(Rav Papa): They do not argue. Rav discusses a field that is not normally planted, and Shmuel discusses a field that is normally planted.

Rav did not say so explicitly. It was inferred from the following case:

David planted Levi's field without permission. Rav said that he must estimate and pay. Levi said 'I do not want it.' Rav said that he must estimate and pay, but David has the lower hand. Levi refused.

Later, Rav saw Levi guarding the crops. Since he showed satifaction that it was planted, he forced him to give David the upper hand.

(R. Yakov): If Reuven planted young trees in a field, he may not take them back.

Question: Why must he leave the trees?

Answer #1: It is for the settlement of Eretz Yisrael.

Answer #2: It weakens the land.

These answers argue about a field in Chutz la'Aretz.

Rishonim

Rif and Rosh (8:23): If Reuven built on Shimon's land, and Shimon fenced or guarded it, we force him to pay for the full building, and Reuven has the upper hand. We learn from the case of David and Levi. This is better than what Rav Hai Gaon wrote in Sefer ha'Mekach. Even if it would be a Safek, the claimant cannot make the defendant pay due to a Safek. All the more so here, for what I wrote is more reasonable!

Rambam (Hilchos Gezeilah 10:4): If Reuven planted Shimon's field without permission, if the field is normally planted, we estimate how much people pay to plant such a field, and Shimon pays this to him. If it is not normally planted, we estimate, and Shimon has the lower hand.

Magid Mishneh: He gets the lesser of the improvements and expenditures. Some say that he gets the smallest estimate of the appraisers.

Rambam (5): If Shimon said 'take your trees and leave', we heed him.

Magid Mishneh: The Rambam discusses a field not normally planted. Then, Shimon prevails. If it is normally planted, Shimon cannot say this. The Ramban and Rashba agree; it seems that the Rif agrees.

Kesef Mishneh: I see no proof that the Rif agrees.

Rebuttal (Ra'avad): We do not say so in Eretz Yisrael.

Magid Mishneh: We can say that this is only when the landowner wants the trees. If it is worth more to plant seeds, this is no claim (to say that he must leave the trees for settlement of Eretz Yisrael).

Rambam (ibid): If Reuven said 'I will take my trees', we do not heed him, for it weakens the land.

Ra'avad: This is why we do not heed him even in Chutz la'Aretz. The other opinion gives another reason why we do not heed him.

Magid Mishneh: The Rambam rules like the latter version.

Rambam (8): If Reuven planted in Shimon's field without permission, and later Shimon guarded the trees, or in another way showed that he is pleased with what was done, we estimate and Reuven has the upper hand.

Rosh (8:22): Shmuel says that we estimate how much people pay to plant such a field, i.e. like professionals who plant for the city. Rav Hai Gaon says that he has the lower hand, and gets the least that the experts appraise.

Bach (1): This means that when the improvement exceeds the expenditures, he gets like the experts appraise. This is included in 'he has the lower hand.'

Rosh (ibid): Levi's field was normally planted. Rav said that David has the upper hand. Levi said that he did not want it planted, and David should take back his trees. We said that Reuven gets like planters receive (if it is normally planted), or he has the lower hand (if it is not normally planted) when Shimon wants to keep the trees. If not, Shimon can say 'take your trees', even if the field is normally planted. He can say 'I prefer to plant grain.' Rav suggested that Levi pay to David the lower amount, i.e. if he (Levi) will agree. When Rav saw Levi guard it, this shows that he wanted the trees. He ruled that David has the upper hand, for the field was normally planted.

(Rosh): Most Meforshim say that the field was not normally planted. This is wrong. If so, we must say that the first time, Rav told Levi that David has the lower hand. If so, why did he say the second time that David has the lower hand'? He already said so! Also, if so, after Levi showed that he wants the trees, why did Rav obligate him to give to David the upper hand?! Surely, when Shimon does not want the trees, he can tell Reuven 'take your trees.' It follows that Rav taught that Reuven has the lower hand when Shimon wants to keep them!

Beis Yosef (CM 375 DH u'Mah she'Chosav Omar): Rashi, the Ramah and, it seems, the Rambam explain that the field was not normally planted. The Rosh said that most explain like this. At first, Rav told Levi to pay Stam. Rav thought that Levi refused because he understood that David has the upper hand. Rav clarified that David has the lower hand. Had Levi said at first 'since it was already planted, I will keep it', David has the lower hand. However, since he said that he does not want it at all, and later we see that he wants it, we say that he wanted it from the beginning. Therefore, it is like a field normally planted, and David has the upper hand.

Drishah (1 DH v'Ayen): The Rosh's proof is valid. This is no defense. However, we said 'Rav did not say so explicitly. It was inferred...' The Rosh was forced to say that this does not refer to Rav's law (about a field that is not normally planted), rather, Rav Papa's teaching, that Rav admits about a field that is normally planted.

Poskim

Shulchan Aruch (CM 375:1): If Reuven planted Shimon's field without permission, if the field is normally planted, we estimate how much people pay to plant such a field, and Shimon pays this to him. If it is not normally planted, we estimate and Shimon has the lower hand.

Beis Yosef (DH u'Mah she'Chosav): Since the Ramah rules like the Rosh, and so is the simple reading of the Rambam, and their reasoning is sound, we rule like this.

Drishah (DH v'Hinei): The Tur brings an opinion that Shmuel's law (he pays what one would pay a planter) is even if Shimon says 'take your trees', for we do not heed him (since the field is normally planted). This is the Rambam. Rav discusses a field not normally planted; then, we heed him. The Tur also brings the Rosh, who holds that in every case we heed him. The Ramah also says so, but he holds that Reuven has the upper hand if he showed that he wants the trees, or even Stam (he did not reveal if he wants them) if the field is normally planted. Rav and Shmuel argue about Stam. The Rosh said that most hold like this, but he refuted them.

Shulchan Aruch (2): If Shimon said 'take your trees and leave', we heed him.

Gra (2): Most Poskim learn this from the episode. Levi refused to pay anything, and Rav was quiet, until Levi showed that he wanted the trees. Most say that this is only for a field not normally planted. The Rosh says that it is in any case. The Nimukei Yosef explains that Rav did not know whether it is normally planted, therefore, the first time he did not specify that David has the lower hand.

Shulchan Aruch (ibid): If Reuven said 'I will take my trees', we do not heed him, for it weakens the land.

SMA (5): The Ir Shushan explains that uprooting the trees would weaken the land. Rashi explains that the trees already weakened the land. It seems that the Rambam and Mechaber agree.

Rema: However, if Shimon said 'take your trees and leave', Reuven may do so, even if Shimon later retracted.

Gra (6): This is like one who said 'swear, and I will pay.' He cannot retract (Shulchan Aruch and Rema, CM 22:3).

Shulchan Aruch (3): If Reuven planted in Shimon's field without permission, and later Shimon guarded the trees, or in another way showed that he is pleased with what was done, we estimate, and Reuven has the upper hand.