Gemara

(Mishnah): We distance a tree 25 Amos from a city. If the city was there first, we cut it without paying the owner. If the tree was there first, we cut it, but we must pay. If we are unsure which came first, we cut it and do not pay.

Question: Why is this different than a tree near a pit (he may not cut when in doubt)?

Answer: If the tree preceded the pit, it may not be cut. When in doubt, we do not cut. Here, either way we cut the tree, so we may cut it. We are in doubt if we must pay for it. To collect money, the owner must bring proof.

Bava Metzia 35a (Chachamim of Neharda'a): If Beis Din allowed a lender to collect a field for payment, the borrower can recover his field if he pays the debt within one year.

(Ameimar): I am from Neharda'a. I hold that he can always redeem his field!

Question: From when may the lender eat the Peros of the land he collected?

Answer #1 (Rabah): It is from when he receives the Adrachta (a document authorizing him to take Reuven's property for payment).

Answer #2 (Abaye): (It is from the day the Adrachta is signed.) When witnesses sign a document, the receiver acquires immediately.

Answer #3 (Rava): It is from when the days of announcement (to sell the field) are over.

Rishonim

Rosh (Bava Metzia 3:4): Shuma Hadar only when the creditor did not improve the land and it did not rise in value. It is unreasonable that a borrower can wait forever until the value rises, and then he will redeem it. This would cause Ne'ilas Deles (deter people from lending)!

Rebuttal (Bedek ha'Bayis CM 103 DH v'Ein): The Poskim did not distinguish this way. The Gemara says 'Shuma Hadar', which connotes even if the price rose. There is no Ne'ilas Deles, for the creditor eats the Peros as long as he has it.

Magid Mishneh (Hilchos Malveh 22:17): If the land declined in price, I say that he cannot force him to return it for less than the debt. If the land rose in price by itself from 100 to 200, the creditor gets only 100. If he would get 200, it would look like Ribis for his money.

Rebuttal (Bedek ha'Bayis CM 103 DH uv'Hukra): If it looks like Ribis, why do we permit the creditor to eat the Peros? Rather, we are not concerned for this, for it is a full sale. If the law is true, it is due to "v'Asisa ha'Yashar veha'Tov", we do what is good for the borrower.

Poskim

Shulchan Aruch (CM 103:9): If Beis Din took land to pay Reuven's debt to Shimon (Rema - Metaltelim are not returned), whether the land was with Reuven or with Ploni who bought it from Reuven, and later Reuven or Ploni or their heirs obtained money and paid Shimon, we remove Shimon from the land, even if he had it many years. If the creditor destroyed or built and did not improve the property through this, he does not receive his expenses.

Gra (21): This is like one who spent money on his wife's property (Kesuvos 80a) or one who bought a field from one whom he knew had stolen it (Bava Metzia 15b).

Beis Yosef (DH Kasav Ba'al): Sefer ha'Terumos says that if the creditor destroyed or built, even if it seems that he did not improve the property through this, he receives his expenses. The Magid Mishneh says that some disagree. Since it is a Safek, the Nitva (defendant) need not pay.

SMA (15): The Shulchan Aruch rules that the borrower is Muchzak in the land, for it was his and he can redeem it when he wants. The lender claims his expenses. It holds that the lender is Muchzak, for Beis Din gave the land to him and he eats the Peros. Chachamim gave to the borrower the right to buy back his land; he is the claimant. Due to the Safek, he must pay the expenditures to take back his land.

Taz (DH Eino): The Shulchan Aruch learns from Bava Basra. When a tree is too close to the city, in any case it should be cut, so we cut it. We are unsure whether or not we must pay, so we do not pay. Here also, in any case Shuma Hadar. We are unsure whether or not we must pay, so we do not pay.

Shulchan Aruch (ibid): If it improved due to the expenditures, he swears and receives like one who worked on another's field with permission.

SMA (16): He gets his expenditures and part of the improvements above the expenditures, like one who worked on another's field with permission, who gets even Shevach ha'Magi'a l'Kesafim (Peros ready to be detached and carried on the shoulder). He does not get all the Shevach, for Shuma Hadar.

Rebuttal (Shach 12): He is not a partner, who gets a share! Rather, he receives like people hired to plant, like one who worked another's field with permission.

Gra (26): He is like a Bar Metzra who buys the field after someone else bought it and improved it (175:6).

Shulchan Aruch (ibid): If it improved by itself, e.g. the price rose, he receives only the debt.

Gra (27): We say so about Bar Metzra (108a), and also in the Havah Amina that one Shuma Hadar applies to rings (35a).

SMA (17): The Tur discussed when it rose in price, for if soil was deposited on it or a date tree became thicker, since the Shevach is recognizable, the creditor would receive it. He would not need to swear, for we can see the Shevach. Alternatively, in this case he obviously does not get Shevach, for it is due to the Mazal of the lender. The Tur and Shulchan Aruch teach about a rise in price, which is not due to his Mazal, and one might have thought that he must pay the higher price, lest he wait to buy it when the price will rise.

Shach (13): I say that if soil was deposited on it or a date tree became thicker, also this is like Ribis (if he will pay the Shevach). The Rosh explicitly says so regarding one who sold a field he did not own (Bava Metzia 1:38).

Rema: Some say that if he invested in it and improved it, it does not return at all. If it rose in price, it returns only for the higher price.

Gra (28): This opinion holds that it is unlike Bar Metzra, for there the buyer is like a Shali'ach of the neighbor.

SMA (18): This is even if the borrower agrees to pay the Shevach, because the creditor acquired through Shinuy. We do not say so when it improved by itself, for this was due to the borrower's Mazal.

Shach (14): I say that if he agrees to pay the Shevach, even the Rosh says that Shuma Hadar, just like when the price rose. If not, what is the Rosh's source to say that it does not return at all when he improved it? Even though it is only for "v'Asisa ha'Yashar veha'Tov", also the law of Bar Metzra is only due to this, and the neighbor can buy even after it was improved. Also, if he cannot redeem it after the creditor improved it, every creditor will improve it a little to ensure that he keep it! This is why the Rosh discussed together active and automatic improvements. The Tur says that the Rosh holds that it does not return when the borrower wants to pay the expenditures. This implies that if he offers to pay all the Shevach, it returns. According to the SMA, if he acquired through Shinuy, the same should apply when the value did not increase! He must say that the Rema argues also in the previous case. This is difficult.

Shulchan Aruch (ibid): If the price declined, the lender does not lose. If the owner or buyer wants to remove him, they must pay the full debt. If Kinyan was done that they will not remove him, they cannot remove him.

Gra (29): Kinyan helps, like for Bar Metzra.