Gemara

(Chachamim of Neharde'a): If Beis Din allowed a lender to collect a field for payment, Shuma Hadar (the borrower can take back his field) if he pays the debt within one year.

(Ameimar): I am from Neharde'a. I hold that Shuma Hadar without a time limit!

Letter of the law, the lender need not return the field. Chachamim obligated him to return it due to "you will do what is straight and good in the eyes of Hash-m."

35a: The Halachah is, Shuma Hadar without a time limit.

Clearly, if Reuven collected Shimon's field, and Levi collected it from Reuven, Levi has no more right to keep it than Reuven (i.e. Shimon can buy it back from Levi);

If Reuven collected Shimon's field, and then sold, gave or bequeathed it to Levi, Levi wanted land, not money (Shimon cannot force Levi to sell it).

If Leah collected Shimon's field and then married Levi, or if Reuven collected Leah's field and then she married Levi and she died, a husband is like a buyer (regarding his wife's property), so the original owner has no right to buy back the field.

(Rav Acha or Ravina): If Shimon willingly gave Reuven his field (to pay his debt), Shuma Hadar.

(The other of Rav Acha and Ravina): We do not say that Shuma Hadar.

The latter opinion considers collection of the field like a sale, for he willingly gave it. The first opinion says that Shimon did not want to give it. He gave it to avoid the embarrassment of going to Beis Din.

Kesuvos 91b: A man owed 1000 Zuz. He had two mansions, and sold each for 500 Zuz to Ploni. The creditor took one mansion, and was about to take the other. Ploni took 1000 Zuz cash and said 'if one mansion is worth 1000 to you, consider one mansion full collection. If not, take 1000 Zuz.'

(Rava): The buyer can force the creditor to do one of these, for the creditor does not lose. He lent 1000 and receives 1000!

Rishonim

Rif and Rosh (3:4): Whenever Rav Acha and Ravina argue, the Halachah follows the more lenient opinion. Therefore, if Shimon willingly gave his field for payment outside of Beis Din, Ein Shuma Hadar.

Rosh: A husband is like a buyer, and he need not sell back the property. If we would say that he is like an heir, it would return. We said that if a creditor collected land and bequeathed it, the recipient wanted land, not money (so Ein Shuma Hadar). That is when the creditor wrote in his lifetime to one of his heirs, similar to a sale or gift. In an automatic inheritance, the heir is like the one he inherits, and he must return land, but land does not return to him. Shuma Hadar only for land, but not for Metaltelim.

Rambam (Hilchos Malveh 22:16): If Beis Din took Reuven's land to pay Shimon, and then gave it to Shimon's creditor Levi, Levi has no more right to keep it than Shimon. If Shimon sold or gave it for a gift or willingly gave it to pay a loan, or died and bequeathed it, it does not return. A husband is like one who bought his wife's property. He need not return it (to one who gave it to her for payment), and it does not return to him (if her creditor takes it).

Magid Mishneh: The Rambam holds that Shuma Hadar also to a buyer. Kesuvos 91b proves this. After the creditor took the house, the buyer returned the money.

Ran (Kesuvos Reish 50a): Some say that this is only before the house was absolutely given to the creditor. After this, Shuma Hadar only to the owner, but not to a buyer. It returns due to "v'Asisa ha'Yashar veha'Tov." The buyer caused his own loss (by buying property with a lien on it). We say that Ein Shuma Hadar to a husband, because he is like a buyer! The Rambam says that we return even to a buyer. The simple reading of our Gemara connotes like this. Perhaps in Bava Metzia, they collected from her before he married her, but if they collected from her after he married her, it would return to him.

Magid Mishneh: The Rashba says in the name of the Ramban that if they acquired (did Chalipin) with him that he will not remove him from the land, he cannot remove him, for he pardoned, like we say about Bar Meitzra (a neighbor, who has rights to buy the land before anyone else).

Rambam (17):

Rosh (Kesuvos 10:3): I saw in the name of R. Yonah that Kesuvos 91b discusses when the mansion was seized without announcement (that it is being auctioned to pay a debt). If it was announced and Beis Din gave it to the creditor, the buyer cannot dispossess him. If you would not say so, there is no end (he could forever redeem it)! The Ramah says that if the creditor sold or gave it to another, Ein Shuma Hadar. "v'Asisa ha'Yashar veha'Tov" does not apply to one who bought land on which a creditor had a lien.

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 for many years.

Shach (10,11): Many say that no other Kinyan is needed. The Ritva says that Shuma does not return to a minor until he matures and knows to be Makneh. This implies that he holds that a Kinyan is required.

SMA (13): It applies only to land, for it is called 'inheritance' and it is intact for generations.

Rema: Some say that a buyer cannot remove the creditor after he collected the land, for Shuma returns only to the original owner when Bnei Chorin (land that the borrower still owned) was collected, but it does not return to a buyer.

SMA (14): Even if the seller has nothing now, the buyer gets back his money when the seller obtains money. He has no claim against the creditor.

Rema: If the borrower willingly gave it to pay a loan, we do not return it to him.

SMA (21): Whenever he needed to bring the borrower to Beis Din, even if he immediately agreed to give the land, this is called Shuma, and it returns.

Rebuttal (Shach 15): Rashi holds oppositely! If Beis Din did not collect from him against his will, this is not considered gong to Beis Din, and Shuma Hadar. Correspondingly, if a Shomer paid willingly, even if the owner needed to take him to Beis Din, (if the thief will be found and will pay double,) Rava says that the owner is Makneh to him the extra payment (Bava Metzia 108a).