Gemara

51b (Mishnah): A Bechor receives Pi Shnayim (two shares) only from what was Muchzak (owned at the time of death) to his father, but not to what was only Ra'uy (destined to come to him, e.g. a debt or inheritance).

He does not receive Pi Shnayim of Shevach (what the father's property increased in value from death until division), nor from Ra'uy, for it says "b'Chol Asher Yimatzei Lo" (at the time of death).

(Mishnah): They do not receive Ra'uy, only Muchzak.

This (is extra. It) includes property inherited from a grandfather.

Bava Basra 124b (Rav Yehudah): A Bechor does not receive Pi Shnayim in a loan due to the father.

He holds like Chachamim (who say that a Bechor gets no extra share in Shevach). One might have thought that a document is considered as if it was collected. He teaches that this is not so.

(Chachamim of Eretz Yisrael): (If they inherit a loan,) the Bechor receives Pi Shnayim in the principal, but not in the interest.

They considered a loan to be as if it were already collected.

(Ameimar): The Bechor receives Pi Shnayim in the principal, but not in the interest.

(Ravina): This is the opinion of Chachamim of Neharde'a (which includes Ameimar and Rabah).

125a (Rabah): I and Rav Nachman explained Chachamim of Eretz Yisrael, but we ourselves hold that he does not get Pi Shnayim even in the principal.

Bava Metzi'a 67b (Rav Papa and Rav Huna brei d'Rav Yehoshua): (Mashkanta is a security of land. If it was agreed that the lender will eats the Peros and return it after a set time,) in a place where Mesalkei (the borrower can redeem his land before the set time), the lender's Bechor does not receive Pi Shnayim in it.

Rishonim

Rashbam (124b DH Aval): Even though the Ribis was written in the document, and a loan is as if it were collected, since he is holding a document, we do not consider the Ribis as if it were collected, even though the time already passed since the father died. This is the ultimate Ra'uy. It never existed. The money of the loan is like getting back the money he lent. The document is Muchzak in his hand in place of his money. It is like a security. All agree that if one lent money and received a security, a Bechor receives Pi Shnayim, for a creditor acquires a security. Also R. Chananel rules like this.

Bach (CM 278:7): Even though a creditor acquires a security only if it was not taken at the time of the loan (Bava Metzi'a 82a), since in such a case he totally acquires, we can say that even a security taken at the time of the loan has a strong lien and it is considered Muchzak, and a Bechor receives Pi Shnayim. Tosfos (Bava Metzi'a 81b) said similar to this and brought proofs.

Question: In a place where Lo Mesalkei, the lender's Bechor receives Pi Shnayim in a Mashkanta. However, a Bechor receives Pi Shnayim in a field that returns in Yovel (Gitin 48b). This is no better than a Mashkanta, which returns to the borrower! We cannot say that the Bechoros of the borrower and lender both receive Pi Shnayim! (It cannot be Muchzak to both fathers.)

Answer (Rashba, Teshuvah 3:198, cited in Beis Yosef CM 278 DH v'Chasav ha'Rashba): Indeed, the lender's Bechor receives Pi Shnayim in the loan, and the borrower's Bechor receives Pi Shnayim in the field, even if he redeemed it after his father died.

Tosfos (67b DH u'Shevi'is): Mashkanta (where Mesalkei) is unlike a loan with a security. Metaltelim are different. The lender holds onto them and acquires a security not taken at the time of the loan, therefore even a security taken at the time of the loan is considered 'of your brother, in your hand.' This does not apply to land.

Poskim

Shulchan Aruch (CM 278:3): A Bechor does not receive Pi Shnayim of property that was Ra'uy to come after his father died, rather, only from what was Muchzak to his father and already came to him, for it says "b'Chol Asher Yimatzei Lo." If a Morish of the father (one whom the father inherits) died after the father, the Bechor and other sons inherit the property equally. Similarly, if the father had a loan (owed to him), they inherit equally.

Beis Yosef (DH v'Ein): The difference between Ra'uy and Muchzak is only regarding a Morish. A Bechor does not receive Pi Shnayim from what was Ra'uy to come to the Morish at the time he died. We are not concerned about the heir. When the Bechor dies, his children inherit his extra share in their grandfather's property, like we said (277:23). If one did Yibum to the wife of his brother the Bechor, who died in his father's lifetime, and afterwards the father died, the Mordechai (Bava Basra 576) discusses how many shares the Yavam receives.

SMA (2): Even though when a Bechor dies in his father's lifetime, the Bechor's children are in place of him and receive Pi Shnayim, there is different, for a son is like the leg (continuation of) his father. It is as if the Bechor is alive and inherits what is found with his father. This is unlike when the Bechor's father died, and the property of the father's Morish was not found with the father when the father died, rather, it was with the Bechor's grandfather. He is not the Bechor of his grandfather. He is the Bechor of his father, and the property was not found with his father.

Shach (2): If the father owned a ship (at sea, this is like a loan).

Shulchan Aruch (7): A Bechor does not receive Pi Shnayim in a loan without a security, but he receives Pi Shnayim in a loan with a security. It is like Muchzak, even if the security was taken at the time of the loan.

Beis Yosef (DH u'Mah she'Chasav Afilu): Maharik (145) says that the Rashbam, R. Chananel, Mordechai and R. Baruch say that a Bechor receives Pi Shnayim in a loan with a security, and it seems that this is even for a Stam loan with a security in which the security was taken at the time of the loan.

SMA (20): The Levush says that it is because he acquired the security. This is wrong. He does not acquire a security taken at the time of the loan, just it is as if the lender collected the loan.

Shach (7): The Levush says like the Rashbam and Mordechai, and like the Gemara says! I said (72:9) that one does not acquire at all a security taken at the time of the loan. We cannot say that since he fully acquires a security taken not at the time of the loan, he partially acquires a security taken at the time of the loan. Even so, the law here is correct. The lender holds a lien on the loan, so it is called Muchzak, and not Ra'uy. The Ra'avad says that a Bechor receives Pi Shnayim from the land itself of a Mashkanta in a place where Mesalkei, even though the lender has a mere lien on it. This suffices to be called Muchzak. The Levush, Rashbam... mean that the lender acquires a lien in the security.

Shulchan Aruch (ibid.): In a place where the lender eats the Peros of Mashkanta for a fixed number of years, and Lo Mesalkei, a Bechor receives Pi Shnayim. In a place where Mesalkei, this is only a loan, so a Bechor does not receive Pi Shnayim.

Bach (DH u'Mah): In a place where Mesalkei, this is only a loan, so a Bechor does not receive Pi Shnayim.

Beis Yosef (DH v'Ha): Seemingly, in a place where Mesalki, a Bechor does not receive Pi Shnayim even in the field itself. However, the Ra'avad explains that this is only in the Peros he eats from it.

SMA (23): I do not know why the Mechaber and Rema did not mention the Ra'avad's opinion.

Bach (DH u'Piresh): The Ri mi'Gash (Bava Basra 125b DH Ha) explains that even in a place where Mesalkei, if the time finished in the father's lifetime, it is no longer like a sale. Now it is like a loan, so a Bechor does not receive Pi Shnayim. However, the Rashba (125b DH u'Mashkonos) wrote that whenever he was unable to take back his land, even where a Stam Mashkanta is for a year, it is like a sale for a year, and it does not cease to be like a sale, even if the time came in the father's lifetime. Therefore, a Bechor receives Pi Shnayim, for he is Muchzak in the property. When the borrower comes to redeem the land, it is as if he buys it back. We need not say that this applies in a place where Lo Mesalkei.

Nesivos ha'Mishpat (Bi'urim 7): I say that they do not argue. The Ri mi'Gash discusses when he collects from the Peros. Once the time came, the lender is not Muchzak in them. The Rashba discusses when he collects the land itself or money. The lender acquired the land until the borrower buys it back. It does not matter whether this is before or after the time. His Kinyan did not lapse!

SMA (22): After the time, it is as if it was redeemed and removed from the lender.

Rema: It is called Muchzak also regarding the borrower. His Bechor receives Pi Shnayim when it returns to the borrower.

SMA (23): This is not only if the time already came. Even if the lender died before the time and his Bechor receives Pi Shnayim, it is called Muchzak regarding the borrower's heirs.