Gemara

10b (Mishnah): All documents written by Nochri courts, even though Nochrim signed them, are valid, except for Gitei Nashim or freedom;

The Mishnah does not distinguish between documents of sale and gift.

Question: Granted, documents of sale are valid. The giving of the money made the acquisition, the document is merely a proof. The judges would not write a document if the money was not given, for this would damage their reputation.

Why is a document of a gift valid? (If Nochri courts made it,) it has no power to transfer property!

Answer #1 (Shmuel): The law of the kingdom (that court documents transfer property) is binding.

Answer #2: The Mishnah should say 'except for documents like Gitei Nashim (i.e. that are not merely proofs, but change ownership or status).

Bava Basra 55a (Rabah citing Shmuel): The law of the kingdom is binding. In Persia, a Nochri makes a Chazakah after 40 years (if Levi buys from a Nochri with such a Chazakah, Yehudah cannot claim 'the land was stolen from me').

Rishonim

Rif (4a): Shmuel answered that Dina d'Malchusa Dina. Alternatively, the Mishnah should say 'except for documents like Gitei Nashim (i.e. anything that does not acquire according to Torah law, but acquires according to their law, we do not accept or collect with it, even if it was made in their courts.

Rosh (1:10): The two answers do not argue. Throughout Shas we say that Dina d'Malchusa Dina. We explain the Mishnah in two ways. If the king enacted that every document must be signed in the courts, after the court knows that it is true, we rely on them. Even if they are suspected to take bribes, if the money were not handed over in front of them, they would not sign Sheker. If so, even gift documents are valid. If there was no such enactment, the Mishnah is Machshir documents except for those like Gitei Nashim.

Rambam (Hilchos Malveh 27:1): Any document signed by Nochrim is Pasul, except for a sale or loan document. It is Kosher if the money was given in front of them and they wrote this in the document. Documents of admissions, gifts, compromises or pardon in which the witnesses are Nochrim are worthless. My Rebbeyim ruled that even their loan documents in which the money was given in front of them are Pasul. We are Machshir only documents of sale in which the money was given in front of them. I disagree.

Magid Mishneh: Most Ge'onim rule like the latter answer, that all court documents are Kosher, except for those like Gitei Nashim, i.e. they create a Kinyan or lien. They explain that d'Malchusa Dina only for things that benefit the king regarding taxes and his laws. His law is not binding for matters between people. The second answer says 'perhaps Dina d'Malchusa does not apply to such matters, and the Mishnah means 'except for those like Gitei Nashim.' There is no solid proof against this.

Rebuttal (Ran DH v'Ibo'is): Shmuel taught that in Persia, a Nochri makes a Chazakah after 40 years due to Dina d'Malchusa. The king has no benefit from this! Rather, the second answer is that there was no royal enactment (like the Rosh above), and all documents like Gitei Nashim are Pasul.

Defense #1 (Bedek ha'Bayis CM 68 DH u'Mah): According to the Nimukei Yosef, there is no proof.

Chidushei Hagahos (1): The Nimukei Yosef (Bava Basra Sof 29a) first explains like the Ran, but concludes 'we say Dina d'Malchusa is binding, not Dina d'Melech, because it applies only to laws that all kings make, but not to what a king takes by force.' The Beis Yosef holds that it is not a law of all kings to be Machshir a worthless document.

Defense #2 (Gra 68:19): The Rambam holds that the law of Chazakah benefits the king, in order that he will get his Chok (taxes).

Poskim

Shulchan Aruch (CM 68:1): Any document signed by Nochrim is Pasul, except for a sale or loan document. It is Kosher if the money was given in front of them and they wrote this in the document.

Beis Yosef (DH Shtarei (2),...): We follow the Rambam against his Rebbeyim and the Ra'avad, because the Ba'al ha'Itur, Ramban, Rashba, Rosh and Ran hold like him. The Gemara asked only why sale documents are Kosher.

Rebuttal (Shach 3): There is no connotation that they hold like the Rambam. The Ran holds like the Ra'avad! A loan document is different than a sale document. Since he does not collect from Meshubadim and there is no Kol, he was not concerned to leave it with the lender. He can say that he paid it.

SMA (3,4): Court documents help when they are merely a proof for the transaction. Even if the seller denies it, we rely on courts like we rely on Beis Din.

Shulchan Aruch (ibid.): Gift documents in which the acquisition is through the document and Nochrim signed, or admissions in which Ploni admitted that he owes Almoni, and compromises (Rema - or pardon), even if they meet all the above conditions (and they were given in front of Yisrael witnesses), they are worthless.

Beis Yosef (DH u'Motzasi): The Rivash (142) was astounded that the Rambam disqualifies documents of pardon. He suggested that the Rambam wrote this inadvertently, but does not really disqualify them. The Rambam also disqualifies admissions, i.e. admission to a debt and the document creates a lien. Admission of payment is valid. R. Yonah says that admitting in front of a judge is like saying 'you are witnesses against me.' We must say that the Rambam disqualifies only when he did not admit in front of Yisrael witnesses.

SMA (11): Admissions do not help, because one must say that he wants those who hear to be witnesses to his admission (Rosh), or he must make a full admission in front of them (Rambam), and courts are not strong enough for this.

SMA (12): The Mechaber disqualifies 'an admission that he owes', but not an admission that he was paid. The latter merely releases a lien; it is Kosher.

Taz (DH Shitrei): Some disqualify admissions because only can say that he was joking. This is wrong. Admission in front of a Kosher judge is like admission in front of Yisre'elim. Rather, there are differences our law and Nochri law about what is considered admission or pardon. We can rely on them only if they wrote Ploni's words. This is why the Shulchan Aruch said 'Ploni admitted that he owes Almoni.'

Gra (15): The Mechaber omitted pardon because it does not need a Kinyan. The document is only a proof. The Rambam holds that only documents of loans and sales are Kosher, for these are major matters and judges would not ruin their reputation about them. Other documents are Pasul, even if they are mere proofs. They do not mention that money was given; judges might lie about them.

Rema: Some say that admission documents are like loan documents, and also documents of pardon are Kosher if done in the courts.

SMA (14): We must say that the pardon is done through the document. If it says that he pardoned him in front of the judges, this is like any document of proof, and all agree that it is Kosher.

Rebuttal (Shach 8): The Rema (below) brings an opinion that it is Kosher. If the pardon were done through the document, this would be like a sale document, and all would agree that it is Pasul! The Rivash (cited in Beis Yosef above) says that it is Kosher due to the SMA's reason.

Gra (17): This opinion distinguishes only between documents of proof and of Kinyan. The Rambam rejects it, for the Gemara did not distinguish like this, rather, between documents of sale (in which money was given) and gifts!

Rema: Also, if a gift was acquired through Chalipin or Chazakah, just he admitted in the court and the document is a mere proof, it is valid. Whenever the king's law is that everything must be written in the courts, even gift documents are valid due to Dina d'Malchusa, and all the more so where the custom is to accept such documents (even if it is not the king's law).