Gemara

175a (Mishnah): If Reuven lent Shimon and wrote a document, he can collect even from Meshubadim. A Milveh Al Peh (for which no document was written) is collected only from Bnei Chorin (property that the borrower still owns).

175b (Ula): Mid'Oraisa, whether or not a document was written for a loan, the lender may collect from Meshubadim (permit that the borrower sold after he borrowed).

This is because Shibud (the lien on a borrower's property to pay the loan) is mid'Oraisa. (A sale does not uproot the lien.) Chachamim enacted that one may not collect a Milveh Al Peh from Meshubadim to protect buyers from losing.

(Rabah): Mid'Oraisa, whether or not a document was written, the lender does not collect from Meshubadim.

Shibud is not Mid'Oraisa. Chachamim enacted that a loan document is collected from Meshubadim due to Ne'ilas Delet (lest people be dissuaded from lending).

(Rav and Shmuel): A Milveh Al Peh is not collected from heirs or buyers.

(R. Yochanan and Reish Lakish): It is collected from heirs and buyers.

They hold that Shibud is mid'Oraisa. Rav and Shmuel disagree.

(Rav Papa): The Halachah is, a Milveh Al Peh is collected from heirs, due to Ne'ilas Delet. It is not collected from buyers, because it does not become known.

Kidushin 13b (Rav Papa): The Halachah is, a Milveh Al Peh can be collected from heirs, because Shibud is mid'Oraisa. It can not be collected from Meshubadim (property he sold after he borrowed), since buyers were unaware.

Kesuvos 85a: A man had deposited documents with a woman to whom he owed money. After his death, his heirs requested the documents. She said that she seized them when he was alive, to be collateral for my debt.

Rav Nachman: If you have no witnesses that he asked for his documents, and you refused to return them, this is like seizure after death. It doesn't help.

92a (Rami bar Chama): Reuven sold a field to Shimon with Achrayus, and Shimon wrote a document saying that he owes the money. Reuven died, and Reuven's creditor took the field from Shimon. Shimon persuaded him to take the money owed in place of the field.

Reuven's children can say 'our father left Metaltelim with you (a debt), and a creditor cannot take Metaltelim of orphans.' (You did not pay our debt, therefore you still owe to us. Also, you cannot collect the Achrayus from us.)

Rishonim

Rif and Rosh (Bava Basra 82b and 10:49): The Halachah is that Shibud mid'Oraisa. Rav Papa, who is Basra, says so in Kidushin. Here, Rav Papa taught that a Milveh Al Peh is collected from heirs due to Ne'ilas Delet, but not from buyers, for it does not become known. He does not contradict himself. He did not say that we collect from heirs due to Ne'ilas Delet to teach that it is only an enactment. Rather, he explains why Chachamim left the mid'Oraisa law in effect, and did not enact to protect buyers.

Rif (Kesuvos 42b): Nowadays a creditor has a lien on Metaltelim, so he collects from orphans' Metaltelim whether or not he seized them.

Rambam (Hilchos Malveh 11:4): A Milveh Al Peh can be collected from heirs, but not from buyers. Since there is no publicity, one may not be Toref (take from buyers). A loan document can be collected from buyers, for it has publicity. The buyer caused his own loss by not asking until he found out that the property is Meshubad. Mid'Oraisa, all the borrower's property is Meshubad to the lender.

Rosh (Kesuvos 9:13,14): Rav Papa says that paying creditors is a Mitzvah, and therefore minors are exempt. This implies that we force adults, i.e. when they inherited land. If they inherited Metaltelim, it is a Mitzvah but we do not force them. Rashi says that we do not force people to do Mitzvos mid'Rabanan. This is wrong. Rather, Beis Din does not force for Mitzvos, such as Kivud Av, for which the reward is explicit in the Torah. Latter Rabanan enacted that a creditor collects Metaltelim (of orphans). Creditors rely on Metaltelim like on land, so we force even if they inherited Metaltelim. This is the practice.

Ran (Kesuvos 50a DH Mitzvah): It is a Mitzvah for orphans to pay their father's debt due to Kivud Av. We do not force them, because it is not an explicit Mitzvah like Sukah or Lulav. It is mid'Rabanan.

Poskim

Shulchan Aruch (CM 107:1): It is a Mitzvah for heirs to pay their father's debt. We force them, just like we would force their father. This is when they inherited land. If they inherited only Metaltelim, we do not force them to pay the debt from the Metaltelim, but it is a Mitzvah for them to do so.

Gra (1): When they inherited land, the Mitzvah is not to honor their father. Rather, they must pay like their father needed to, for Shibud is mid'Oraisa.

Tur: Mid'Oraisa, Metaltelim of orphans are not Meshubad to a lender, even if they are deposited with the lender or others.

Chidushei Hagahos (1): This refers to Metaltelim that the father deposited with the lender, and in his lifetime, the lender did not intend to be Tofes (seize them for collection). We learn from the deposited documents (Kesuvos 85a) and the one who owed for a field bought with Achrayus (92a). They could not keep them, for they did not intend for Tefisah.

Shulchan Aruch (ibid.): This is letter of the law. Ge'onim enacted that a lender collects from inherited Metaltelim. Therefore, nowadays we force them to pay his debt, even a Milveh Al Peh, even if the father acquired the Metaltelim after he borrowed, and he did not write 'what I will acquire' (will be Meshubad). This is because an heir is in place of his father. They must also pay his debt from a loan that others owed to him, whether they collected land or money.

SMA (4): They must pay even though there are two weaknesses, i.e. the debt is Metaltelim, and it was not Muchzak with the father when he died.

Gra (7): Even though a debt is considered Ra'uy (not Muchzak) regarding a Bechor (he does get a double portion from it) and a husband, and a creditor does not collect from Ra'uy, he collects from a debt.

Shach (3): Tosfos (Bechoros 52a DH b'Ra'uy) and the Rosh (Kesuvos 8:13) say that the creditor's creditor can collect from inherited Metaltelim, through Shibud d'R. Noson. The Ra'avad says that we do not apply two stringencies; perhaps this if for a Yavam, but not for a creditor. Rashi says that we do not apply two stringencies for a Kesuvah, which is mid'Rabanan. He could agree about a creditor. The Ba'al ha'Itur and Ramban say so about a creditor.

Shulchan Aruch (ibid.): If they inherited land and Metaltelim, and the document does not specify that he will collect Metaltelim in his lifetime of after death, and the lender wants Metaltelim, the heirs can choose to pay land.

SMA (6): This is because letter of the law, the lender does not collect at all.

Shach (4): The Bach (105:4 DH u'Mah) says that seizure of Metaltelim helps if the orphans wanted to pay land. This is wrong.

Shulchan Aruch (ibid.): If they did not inherit anything from their father, there is not even a Mitzvah to pay his debt.

Rema: If one inherited a small amount, he need pay only what he inherited.