Gemara

98b (Rabah): If Reuven burned Shimon's loan document, he is exempt. He can say 'I burned a mere piece of paper!'

Question (Rami bar Chama): If witnesses know what was in the document, they can write another document! If no witnesses know what was in the document, how could we possibly obligate Reuven to pay?

Answer (Rava): The case is, Reuven trusts Shimon to say what was written.

(Rav Dimi bar Chanina): R. Shimon and Chachamim argue about Rabah's law. R. Shimon holds that Davar ha'Gorem l'Mamon (something that has no intrinsic value, but if it is lost, one will lose money) k'Mamon Dami (it is like property with value). He obligates for burning documents. Chachamim hold that Davar ha'Gorem l'Mamon is not k'Mamon. They exempt.

Question (Rav Huna brei d'Rav Yehoshua): R. Shimon said (that it is k'Mamon) only regarding something that was originally worth money, like Rabah's case (Reuven stole Shimon's Chametz before Pesach, and Levi burned it after Pesach. R. Shimon obligates him, for now Reuven cannot return the Chametz itself, and must pay for his theft.) We never find that R. Shimon obligates for something that never had intrinsic value!

(Ameimar): According to the opinion that obligates for Garmi (causing damage), he must pay the full value of the document. According to the opinion that exempts for Garmi, he pays the value of the paper.

A case occurred, and Rafram forced Rav Ashi to pay the full value.

Rishonim

Rif: Rabah exempts one who burned another's document. However, the Halachah follows Ameimar, who forced Rav Ashi to pay fully. This is only when the one who burned agrees about what was written in the document, and because he burned it the lender cannot collect.

Rambam (Hilchos Chovel u'Mazik 7:9): If Reuven burned Shimon's document, he must pay the entire debt written in the document. Even though it is has no intrinsic value, he caused a loss of money. This is only if Reuven admits that it was a validated document, and because he burned it Shimon cannot collect. If not, he pays only the value of the paper.

Rosh (9:13): If witnesses know what was in the document, they can write another document. Even though written testimony is invalid, a signed document is like testimony that was accepted in Beis Din. It is as if the latter witnesses testify that a Beis Din obligated the borrower to pay. The lender is not believed to say what was in the document. An enactment was made (to believe the victim) when property with intrinsic value was burned. Here it is only Gerama. The Gemara was unsure whether an enactment was made to believe a victim of a Moser. Here, surely no enactment was made, for he burned a mere piece of paper.

Yam Shel Shlomo (9:19): Really, here it is Garmi. In any case, Moser is more stringent. If Reuven says how much the document was for, he swears and pays only that. If he is unsure, he swears that he is unsure. However, if he knows that there was a debt, but is unsure how much, he is Modeh b'Miktzas. Since he cannot swear, he must pay.

Rebuttal (Shach CM 386:15): The Gemara never answered whether an enactment was made for Moser. The same applies to burning a document. It is a Safek, so we cannot obligate the one who burned to pay.

Poskim

Shulchan Aruch (CM 386:2): Similarly, if Reuven burned Shimon's document, he must pay the entire debt that was in the document.

Question (Shach 10): Sefer ha'Terumos (brought in Beis Yosef DH v'Chosav Ba'al) says that if Shimon says that the document was for 100 and Reuven says that it was for 50, he must swear due to Modeh b'Miktzas. If he is unsure if it was more than 50, since he cannot swear he must pay. This is difficult. We do not swear about documents (Shevu'os 42b)! Perhaps that Mishnah in Shevu'os exempts Garmi, or discusses when Shimon can collect his loan. However, the Poskim bring Stam that we do not swear about documents, i.e. in every case.

Answer (Shach): I have no good answer. It seems that Sefer ha'Terumos equates Garmi and Davar ha'Gorem l'Mamon, and rules like R. Shimon. He says that also the Ra'avad says that Davar ha'Gorem l'Mamon depends on Garmi. Most Poskim, including the Rif, Rambam, Magid Mishneh and Rosh, rule unlike R. Shimon, so the Halachah does not follow Sefer ha'Terumos. The Ramban (Dina d'Garmi) obligates one who burns a document to pay due to Garmi, even though the Halachah is unlike R. Shimon, and we do not swear about documents because they have no intrinsic value. He says that one does not swear mid'Oraisa about Garmi. This is unlike the Rashba and Teshuvos Maimoniyos. Also the Rambam says that there is a Shevu'ah mid'Oraisa about Moser, who is liable only due to Garmi. The Yam Shel Shlomo (Bava Kama 9:16) obligates a Shevu'ah when Reuven is unsure how much was in the document, like the opinion in Tosfos (that also Chachamim of R. Shimon obligate for Garmi). However, the other opinion in Tosfos (Bava Kama 71b DH v'Savar) is primary, that his Chachamim admit only about principal, but not for Kefel. If so, they also exempt from swearing. Also, I hold that Garmi is mid'Rabanan, so there is no Shevu'ah mid'Oraisa about it.

Shach (13): He pays as much as one would pay to buy the debt, just like one who sold a document and pardoned it. This is like the Rosh (Teshuvah 69:1). We cannot distinguish these.

Shulchan Aruch (ibid.): Even though the document is not intrinsically money, he caused him to lose money. This is only if Reuven admits that the document was validated and how much it was for, and because he burned it Shimon cannot collect. If Reuven does not believe (that it was validated, or how much was written), he pays only the value of the paper.

Yam Shel Shlomo (9:19): If Reuven seized the document before burning it, all agree that he is liable. This is no worse than Garmi. Rabah admits that one who seized a coin and threw it to the sea is liable.

Rebuttal (Shach 13): Rabah exempts Stam, i.e. (in any case,) even if he seized it and burned it. Regarding throwing a coin to the sea, Rabah distinguished between whether or not he seized it first. He did not distinguish regarding burning documents. Also, land and documents cannot be stolen (Bava Kama 117b). One who stole land is exempt even if it was flooded, for it was always in the owner's Reshus. The same applies to documents.

Rema: This is when witnesses do not know what was in the document. If there are witnesses, they can write another document!