Gemara

Question (Rav Huna bar Chinena): If one sold himself to a Nochri, can he be partially redeemed? Do we learn a Gezeirah Shavah "Ge'ulaso-Ge'ulaso" from (one who sold) Sedei Achuzah (a field he inherited from his ancestors)? Just like a Sedei Achuzah cannot be partially redeemed, also a slave sold to a Nochri!

Or, perhaps partial redemption applies when this is a leniency?

Answer (Rav Sheshes): We expounded "he will be sold" - entirely, not partially. Likewise, "he will be redeemed" entirely, not partially.

(Abaye): Partially redemption can be a leniency or a stringency. It is a leniency when a slave was sold for 200, decreased in value to 100, paid 50 for half redemption, and rose in value to 200. (If partial redemption works, he pays 100 more to compete the redemption. If it does not work, he must pay another 150.)

It is a stringency when he was sold for 200, paid 100 for half redemption, and decreased in value to 100. If partial redemption works, he pays another 50 to compete it. If it does not work, it is as if the 100 were a deposit with his master. This suffices to redeem him now!

Question (Rav Huna bar Chinena): If one sold Bayis Ir Chomah (a house in a walled city), can he partially redeem it? Do we learn "Ge'ulaso-Ge'ulaso" from a Sedei Achuzah that was sold? Just like it cannot be partially redeemed, also Bayis Ir Chomah;

Or, perhaps there the Torah said that partial redemption does not work, but here it did not say so (so it works)?

Answer (Rav Sheshes): We learn from R. Shimon that he may borrow money to redeem the house, and he may partially redeem it.

(Beraisa): The repetition "v'Im Pado Yifdeh" teaches that one may borrow to redeem his Sedei Achuzah from Hekdesh, and he may partially redeem it;

R. Shimon says, this is because if one sold his field and does not redeem it before Yovel, it reverts to him in Yovel. Therefore, he may not partially redeem it. If one was Makdish his field and does not redeem it before Yovel, it is given to the Kohanim. Therefore, he may partially redeem it.

(Culmination of answer): Similarly, if one sold his house and does not redeem it within a year, he cannot redeem it. Therefore, he may partially redeem it.

Bava Basra 102b (Mishnah): If one was Makdish his Sedei Achuzah, he can redeem to for 50 Shekalim for a Beis Kor. If there were ditches or rocks, they are redeemed with the field only if they are less than 10 Tefachim deep or tall.

Question: Even when they are 10, they should be like another Hekdesh field!

Rishonim

Rashba (4:290, brought in Beis Yosef YD 172 DH Kosav ha'Rashba): A Stam Mashkanta (the lender eats the Peros of the borrower's field; they did not discuss partial redemption) can be partially redeemed. The Gemara said that Sedei Achuzah cannot be partially redeemed. It asked about Eved Ivri and Bayis Ir Chomah. If we do not learn a Gezeirah Shavah, we say that only Sedei Achuzah cannot be partially redeemed, but Bayis Ir Chomah can be. This implies that a regular Mashkanta can be partially redeemed. If it could not be partially redeemed, there would be no source to say that a Bayis Ir Chomah is different!

Rashba (ibid.): When it is partially redeemed, the lender eats all the Peros until he is fully paid. His lien is on the entire land to pay the entire loan. Abaye taught that partially redemption is a stringency when a slave was sold for 200, paid 100, and decreased in value to 100. If partial redemption works, he pays another 50. If it does not work, the 100 was a deposit, and suffices to redeem him now. Surely, partial redemption works if the buyer agrees. If the seller gets back the Peros, a half redeemed slave should alternate days working for his master and for himself! Why was the 100 a deposit? Rather, partial redemption does not diminish the lien at all. The money is like a deposit. Once, a man stipulated to be paid only in front of witnesses who teach Halachah. He was paid in private, and the money was lost due to Ones. It was a deposit until proper witnesses come (Shevu'os 41b).

Rashba (ibid.): The Rashbam (103a DH Likadshu) says that the Hava Amina was that we can redeem the rocks by themselves, for they are like a separate field. This is difficult. One can always partially redeem a Sedei Achuzah from Hekdesh! Rather, one cannot tell the Gizbar 'here is half the money, return (half) my field', for the Gizbar can say 'you were Makdish the entire field!' Rather, he can force him to accept half the money. In Yovel, half of the field will not go to Kohanim. Likewise, one who gave a field or two for Mashkanta can force the lender to accept half the money, but cannot demand back half the Mashkanta. The Ramban also says so.

Poskim

Rema (YD 172:3): Any Mashkanta cannot be partially redeemed. If the borrower paid part, the lender eats the Peros until he is fully paid.

Shach (22): This is a textual error. It should say can be redeemed, like Teshuvas ha'Rashba, the Ramban and Shulchan Aruch (CM 74:4).

Gra (15): The text is correct. In this sense (to get back half the Mashkanta), it cannot be partially redeemed. According to the Shach, it should say 'in any case, if he partially redeemed...'

Shulchan Aruch (CM 74:4): If Levi wanted to repay a loan bit by bit before the set time, some say that the lender (David) cannot demand to be paid only at once. However, if Levi gave a field or two for collateral, he cannot force David to return half the collateral for half the money.

Beis Yosef (DH u'Mah she'Chosav): Sefer ha'Terumos says so in the name of the Ramban. Another version of Sefer ha'Terumos says that some disagree. He equates securities of Metaltelim and land.

Taz: In a Stam loan, David eats the Peros and deducts them from the loan. He can say that he prefers to work and eat the Peros of the entire field. If Levi gave a security of Metaltelim, David must return half due to Midas Sedom.

Shach (18): David does not eat the Peros, yet he may keep the field to pressure Levi to pay fully. If David eats the Peros, even after full payment he keeps the field until the set time (Sa'if 3), unless the custom in this place is that the borrower can redeem it early.