Gemara

(Beraisa): A Bechor does not get a double share in Shevach (improvements to the property) after the father's death;

Rebbi says, he gets a double share in Shevach.

If they inherited a loan document, the Bechor collects an extra share. If their father's creditor brought a document against them, the Bechor pays a double share;

The Bechor may say 'I will not collect more than one share, nor pay more than one share.'

Chachamim learn from "Lases Lo Pi Shnayim" - the Torah calls the extra share it a gift. Just like a gift takes effect only after it comes to the receiver, also the extra share of the Bechor.

Rebbi learns from this that he has the option not to receive and not pay extra (to a creditor of the father).

Rishonim

Nimukei Yosef (54b DH Garsinan): Even though a Bechor does not receive a double portion of a loan document owed to the father, for it is Ra'uy, he pays a double share of a document against his father, just like he receives a double portion of (Muchzak) property, for there is a lien on all the property to pay the debt. Even though a Bechor has rights in the extra share before receiving it, and he can sell or pardon it, he does not own it like a regular inheritance. Tosfos holds that even if he said 'I do not want it', he can retract, even if his brothers paid money. The Ra'avad holds that it is a gift. A simple pardon suffices. Therefore, he may say 'I will not collect more than one share, nor pay more than one share', for he cannot retract. However, it seems that also regarding the regular share, he can say 'I do not want to deal with it. Authorize him over the property and appraise it.' Any borrower can say this. We do not force him to deal with it. However, he must accept a Cherem that he does not have money to pay. The Rambam and Ge'onim agree.

Hagahos Ashri: If a creditor brought a document against them, the Bechor pays a double share. If the Bechor says 'now I will not collect the extra share, until later, when I want, so now I will pay only one share', he may. This affects the law in a case when the creditor cannot force his brothers, who went overseas. We do not give land to a creditor until we hear the claims. Also, it affects the law when the brothers are minors, and the creditor cannot take them to Din (now).

Rashbam (DH v'Im): Some say that the Bechor can say 'I will not receive a double portion of a loan document owed to our father, and I will not pay a double share of a loan of the document against our father.' This is totally wrong. Creditors do not collect from a loan document owed to the father, for they have no lien against Metaltelim of orphans. Since the Bechor receives a double portion of land, he must pay a double portion of his father's debt.

Poskim

Shulchan Aruch (CM 278:10): If a creditor brought a document against them, the Bechor pays a double share. If the Bechor says 'I do not want to receive an extra share, nor to pay more than one share', he may.

Beis Yosef (DH Yitza): The Rashbam explains that if he pardons his extra share of the entire estate, and will receive like a regular brother, he pays only according to what he receives.

SMA (27): This is because Bechorah is called a gift; it is not his until he receives it. The share of a regular brother is different. He cannot dismiss it by saying 'I do not want it.' He must say that he makes it Hefker.

Shulchan Aruch (ibid): This affects the law in a case when the brothers are minors, or they are not here now. The creditor collects from the Bechor as if he receives only one share.

Prishah (10): When the brothers are minors, he collects from the Bechor the amount that a regular brother must pay. When the brothers are not here now, he collects (up to) the entire debt from the single portion that the Bechor opts to receive.

SMA (28): R. Tam gives another law that this affects. If the Bechor owed money not due to his father, when he pardons his extra share, his creditor cannot collect it from the brothers. This requires investigation (why the Shulchan Aruch omitted this).

Prishah (10): He can totally pardon his inheritance, and his creditor cannot collect at all.

Shulchan Aruch (ibid): Some say that he cannot retract later.

SMA (29): The Tur cites the Ra'avad to say that when he says 'I will not inherit my regular portion and not pay', he can retract later, for when his father died, he acquired his regular portion. Saying 'I will not pay' is not a sale, gift or Hefker. Only regarding the extra share, which is called a gift, once he pardons it he cannot retract. The Shulchan Aruch brought only these last words of the Ra'avad. It holds that one cannot say 'I will not inherit my regular portion, and I will not pay', for a son is in place of his father.

Rebuttal (R. Akiva Eiger): The Ri mi'Gash obligates the son to participate in paying the creditor. The Shulchan Aruch could hold like the Rashba and Nimukei Yosef, that a son cannot say 'I will not inherit. You cannot collect at all now friom my brothers, who are minors', but he can say 'authorize him over the property, and appraise for him.'

Gra (17): The Ra'avad holds that we force a borrower's children to pay his debt only when they have money that they inherited from him. We learned about a Bechor to teach about retraction, but even a regular son can say 'I will not inherit, and I will not pay.'

Teshuvas Pnei Yehoshua (CM 1): A loan document came against Reuven's estate. The Bechor wants to pay it from Ra'uy, so he can inherit a double portion from Muchzak. The other brothers want to pay it from Muchzak, so the Bechor will inherit only one portion from Ra'uy. If the Ra'uy was a loan owed to Reuven, we heed the other brothers. A creditor collects from a loan owed to the borrower due to Shibud d'R. Noson. This is only when he cannot collect from elsewhere. Here, he can collect from Muchzak! Even if the heirs already collected the loan and it is in their Reshus, there was no lien on it to pay Reuven's debt when Reuven died, so the other brothers acquired it all (except for the Bechor's single share). However, if the Muchzak property was flooded, the creditor collects from the Ra'uy. We find that a creditor does not collect from Meshubadim when there are Bnei Chorin, but if the Bnei Chorin were flooded, he collects from Meshubadim. The Rashbam said that creditors do not collect from a loan document owed to the father, for they have no lien against Metaltelim of orphans. Do not say that that is when the creditor wantsto collect land, for his lien is against the land', but when he wants to collect from documents, we heed the Bechor. The Bechor can force the creditor to collect from the documents. The enactment that creditors do not collect from orphans' Metaltelim is to benefit the orphans, so they can decline the enactment. Tosfos and the Rosh hold that a loan not yet collected is not considered Metaltelim of orphans. Why didn't they explain like the opinion the Rashbam rejected? We must say that a creditor has no lien against a loan as long as there is other property.

Noda b'Yehudah (CM 1:34): This case (of the Pnei Yehoshua) is unlike Parnasah (a girl getting a 10th of her father's estate for a dowry), or a Chetzi Zachar document (it says that a daughter will receive half of what a son would receive). There, the Rema (EH 113:2) says that if land and Metaltelim remain, the heirs can choose to pay creditors with land in a case of Parnasah, or with Metaltelim in a case of a Chetzi Zachar document. There, the daughter does not inherit at all. The sons inherit. They are in place of the borrower, so they decide how to pay. A Bechor and a regular son both inherit. One has no more say than the other! We may learn from the Yesh Meforshim (the opinion that the Rashbam rejected). They hold that even though the Bechor gets double from some property, since he does not get double from all of it, he need not pay double. The same applies here. Since some property is Ra'uy, the Bechor need not pay more than other brothers. The Rashbam rejected this opinion only because the creditor cannot collect from loans owed to the borrower, for they are Metaltelim of orphans. Nowadays, we collect from Metaltelim of orphans, so the Halachah follows this opinion. In everyday practice, a creditor collects from debts owed to the borrower. The Shulchan Aruch (CM 107:1) obligates orphans to pay their father's debt from loans owed to their father. A creditor does not collect from loans due to Shibud d'R. Noson. Rather, our primary income is from loans to Nochrim on interest, and a creditor relies primarily on them, just like in certain places creditors rely primarily on ropes (Kesuvos 67a). The Rosh (36:3) says that a Kesuvah can be collected from loans for this reason. However, in the Rashbam's case, the Bechor's pardon helps the other brothers. We cannot learn to a case in which it harms them. Why didn't the Yesh Meforshim learn like the Rashbam, that the Bechor totally pardons his double portion? They hold like the Rashba, that any heir may say 'I will not inherit, and not pay.' If so, the Chidush of Bechor must be that if he pardons any part of his extra share, he pays only one share. Here, there is no reason why the Bechor can force the creditor to collect from the loan. The regular brothers can say that he will collect from Muchzak.