Gemara

(Mishnah): Reuven lent money to Shimon, who is a Kohen, Levi or Oni (poor person). He stipulated that he will keep the Terumah or Ma'aser or Ma'aser Oni that he would have given to Shimon, and deduct its value from the loan. He may do so, without concern lest Shimon died or (if he was poor) became rich.

If he hears that Shimon died, he must ask the heirs for permission to continue the deal. If the loan was made in front of Beis Din, he need not ask for permission.

(Beraisa - R. Eliezer ben Yakov): If Reuven lent money to Shimon, a Kohen or Levi, in Beis Din, and Shimon died, Reuven may continue to keepTerumah or Ma'aser and deduct their value from the loan.

We consider the deal to continue with the heirs, who are also Kohanim or Leviyim. If Shimon was an Oni, and he died, Reuven may continue to keep Ma'aser Oni and deduct its value from the loan.

We consider that other Aniyim of Yisrael agree to continue the deal.

If Shimon became rich, Reuven may no longer keep the tithes. Shimon need not pay back the remainder of the loan.

Chachamim did not enact that Reuven can continue keeping tithes when Shimon becomes rich because this is rare. They enacted for death, for it is common.

(Mishnah): If he hears that Shimon died, he must ask Shimon's heirs for permission to continue the deal.

(Beraisa - Rebbi): The Mishnah discusses heirs who inherited.

(R. Yochanan): He means that they inherited land, not (only) money.

(R. Yonason): Reuven can withhold tithes only up to the value of the land that Shimon left over.

(R. Yochanan): Even if Shimon left over land the size of a needle, Reuven may withhold the full balance of the loan.

This is like a ruling of Abaye, that one may repeatedly collect (parts of) a debt from the same piece of land.

Rishonim

The Rif and Rosh (15b and 3:11) bring our Mishnah verbatim.

Ba'al ha'Ma'or: Rebbi taught that the Mishnah discusses heirs who inherited. He refers to the Seifa, which says that if he lent in front of Beis Din, he need not ask the heirs for permission. A loan in front of Beis Din is like a loan with a document, so it can be collected from heirs. R. Eliezer ben Yakov says that in this case, they can separate on behalf of other people of the Shevet, even if they are not heirs, e.g. Shimon's sister's children inherited him, and they are not Kohanim or Leviyim. R. Eliezer ben Yakov argues with the Mishnah (which allows only due to heirs who inherited).

Rebuttal #1 (Ra'avad, in Kasuv Sham): The Gemara taught Rebbi's law after citing the Reisha of the Mishnah. Also, if R. Eliezer ben Yakov argued with the Mishnah, the Gemara would have said so!

Rebuttal #2 (Milchamos Hash-m): Even outside of Beis Din, it is like a Milveh Al Peh (a loan without a document), which can be collected from heirs! We are not concerned lest a borrower paid before his loan was due. All the more so here we are not concerned (since their father was not obligated to pay at all)! Also, surely a loan in front of Beis Din is like a loan with a document only when the lender had the right to collect! Rather, Rebbi refers to the Reisha. Even though the heirs are obligated, their permission is needed when they inherited little land. They could pay the value of the land, and then the lender would have no way to collect the rest of his debt!

Rebuttal #3 (Rashba 30b Sof DH Ha): The Halachah always follows R. Eliezer ben Yakov. We should not explain that the Mishnah is unlike him!

Rambam (Hilchos Ma'aser 7:7): If the borrower died, the lender cannot continue without permission of the heirs. This helps only if the borrower left land.

Ri Korkus: Tosfos and the Ba'al ha'Ma'or hold that if permission were needed when the heirs inherited land, all the more so it would be needed when they inherited money! The Rambam holds that permission does not help when they inherited money. Since they did not need to pay their father's loan, they are like strangers; the loan was lost.

Question (Tosfos Yom Tov 3:7 DH Tzarich): Nowadays (after Ge'onim enacted) that creditors can collect from Metaltelim of heirs, it helps even if the borrower left Metaltelim. Why didn't the Rambam mention this?

Answer (Yeshu'os Malko, brought in Tziyunim in Frankel Rambam): The Rambam holds that the Ge'onim's enactment was not made in this case.

Milchamos Hash-m: In the Yerushalmi, R. Chanina says that there is no inheritance to an Oni, i.e. his children might not be Aniyim. Since he did not need to repay, also his children are exempt. Therefore, it does not help to ask permission from an Oni's children.

Shirei Korban (18b DH Ein): The Rashba cites this Yerushalmi. We rule like the opinion that it suffices if he left land the size of a needle! Rather, R. Chanina teaches that the Mishnah does not discuss asking permission from an Oni's children, for they are not necessarily Aniyim. The Rambam allows asking permission from an Oni's children. He holds that the Mishnah discusses when also his children are Aniyim.

Rambam (ibid.): It suffices if he left land the size of a needle.

Ri Korkus: This is because one can repeatedly collect from the same piece of land. Some say that it is because Ona'ah (overcharging) does not apply to land, and we can say that it is worth any amount.

Rambam (ibid.): If he lent to him in Beis Din, he does not need the heirs' permission.

Ri Korkus: Beis Din can make things Hefker, therefore it can impose the debt on all Kohanim.

Poskim

Rema (YD 257:5): If Reuven separates Ma'aser and lent to an Oni from his own money, he may consider the loan to be Ma'aser and keep that amount of Ma'aser for himself. This is only if the Oni is still alive. If he died or became rich and he need not pay, for he was an Oni at the time, Reuven may not keep Ma'aser due to him, for we do not separate based on what was lost. One need not be concerned lest the Oni became rich, unless he finds out.

Beis Yosef (DH v'Chosav Od bi'Shmo): Sefer ha'Terumos says that if the Oni died, even if his heirs inherited from him they need not pay anything. Permission from them does not help.

Dagul me'Revavah: This is when the heirs are rich. If they are poor, their permission helps. Perhaps the Yerushalmi means that one must verify that the heirs are poor, for it is not hereditary. Alternatively, it means that one who lends an Oni does not intend to continue if the Oni will die (perhaps his heirs will be rich), therefore even if the heirs are poor, permission does not help.