Gemara

22b (Mishnah): One must distance a ladder four Amos from a dovecote, in order that a marten (a weasel- like animal) cannot jump from it.

(Rav Tuvi bar Masnah): This teaches that Gerama of damage is forbidden.

Rav Yosef had small date trees; bloodletters sat under them. Ravens came to eat the blood of their Kelim, and perched on the trees, and damaged the dates.

Rav Yosef: The bloodletters must leave! Rav Tuvi taught that Gerama of damage is forbidden.

Bava Kama 98a (Rabah): If Reuven threw Shimon's coin into the sea, he is exempt.

This is because the coin is there; it can be retrieved. This is only if the water is clear, so the coin can be seen. This is only if he knocked Shimon's hand. If he took the coin and threw it, he stole it, and he must return the coin to Shimon.

(Rabah): If Reuven rubbed out the image on Shimon's coin, he is exempt.

This is because he did not do (take away) anything, e.g. he pounded it with a hammer. If he filed it smooth, he removed some of the metal, so he is liable.

98b (Rabah): If Reuven burned Shimon's loan document, he is exempt;

He can say 'I burned a mere piece of paper!'

Rishonim

The Rif (Bava Basra 12a) brings the Gemara in Bava Basra.

Rif (Bava Kama 35a): Rabah exempts one who pushes a coin into the sea, makes a groove in the ear of a cow, or burns documents. The Halachah does not follow him for all of these. Rafram forced Rav Ashi to pay the full value of a document that he burned. This is like the opinion that obligates for Garmi.

Rambam (Hilchos Chovel u'Mazik 7:7): Anyone who causes to harm to another's property must pay full damage from his best property, like other damagers. Even though he did not do the final damage, since he is the first cause, he is liable.

Rambam (11): If Reuven pushed Shimon's coin and it rolled into the sea, or flattened Shimon's coin, he is liable.

Rosh (Bava Kama 9:13): Tosfos says that the following are Gerama, and all exempt for it. Leaving a ladder near a dovecote, throwing a Kli off the roof, and then removing the pillows below, putting poison in front of an animal, sending a fire with a Cheresh, lunatic or child, breaching a fence in front of an animal (it left and damaged, or was lost), bending grain in front of a fire, working with a Parah Adumah, deafening someone through scaring him or blowing a Shofar in his ear, fanning a flame and the wind spread it, inciting a dog or snake, bringing Peros into a Chatzer (and the Ba'al ha'Bayis' animal ate them and got sick). Inciting a dog or snake is not certain to damage. One who removes pillows does not touch the object that is damaged. When one sent a Shali'ach with a fire, the Shali'ach did the damage. One who was scared damaged himself by being frightened. Alternatively, Gerama is when the damage is not immediate. The one who obligates for unobservable damage (Gitin 53a) exempts for working with a Parah Adumah or the water with which its ashes will be mixed, because the Isur depends on mere thought, and it is Gerama (even though it is immediate). Really, it is Garmi; that Gemara is like Chachamim, but R. Meir obligates.

Rosh (Bava Basra 2:17): Even though one is exempt for Gerama, l'Chatchilah one can prevent another from causing damage.

Teshuvas Rashba (1052): If Levi paid part of his debt and they left the document with Ploni and he returned it to David, who then collected the full amount, and Ploni admits, he is exempt b'Yedei Adam, for it is Gerama. If David did not yet collect, we excommunicate Ploni until he accepts to compensate Levi for all losses due to the document. We force one to remove all damage of Gerama.

Poskim

Shulchan Aruch (CM 386:1): We hold like R. Meir, who obligates for Garmi. Therefore, if one pushed another's coin until it fell into the sea, or flattened it, even though nothing is missing, he must pay.

Rema (3): Some say that the following are Gerama, and are exempt: one who pushed another's coin to the sea, or flattened the image, a scribe who wrote the wrong amount in a document, one who damaged by giving bad counsel, and a middleman who improperly returned a document. However, we excommunicate him until he removes the damage. The same applies to every Gerama in Nezikim.

Beis Yosef (CM 386 DH Kosav ha'Rashba): The Rashba (3:76) exempts one who returned a paid document to the lender, for there is no immediate loss. Writing the wrong amount in a document is Gerama. If one sold land to a Nochri, the Rosh says that Beis Din makes him compensate his neighbor (for damage that the Nochri does). The majority exempt, for it is Gerama.

SMA (8,9): We excommunicate before the damage was done or the loan was collected. However, the Rema (below) says that some fine for every Gerama that is common, i.e. even after the damage was done. The Rashba (1052) obligates one to pay for damage that occurred after he was excommunicated. This is like the Ra'avad. The Ran (Bava Metzia 108b DH Ela) says that we excommunicate him until he removes the damage. The Rosh (Bava Metzia 9:28) obligates paying all the damage. It seems that if he died before accepting to remove the damage, he (i.e. his heir) is exempt.

Shach (7): The primary opinion exempts for everything that Rabah exempts, except for burning a document. One who pushed a coin to the sea or flattened it is exempt only if he never picked it up.

Rema (ibid.): Some fine for every Gerama that is common.

Shach (24): I disagree. Who will say which Geramos are common? I say that we fine only where we find that Chazal did. Garmi is a mere fine mid'Rabanan. We should not add to it!

Rema (ibid.): If Reuven lent money (on interest) to a Nochri (Tony) and took securities, and Shimon told Tony that he will lend to him with less (securities) and Tony returned the money to Reuven, Shimon is exempt, for it is Gerama, but he is called a Rasha.

SMA (10): R. Yerucham exempts Shimon letter of the law even if he told Tony to return the money. A case occurred in which Ricky contracted to buy whiskey from David. Then, Ricky went to Moshe to try to get a better deal and retract from his deal with David. David told Moshe and asked him not to sell to Ricky, but Moshe did not heed him. We do not punish Moshe, for Ricky went to Moshe by himself. Also, perhaps he will also honor his contract with David! R. Yerucham exempts even Levi who told Bart 'take this money and pay your debt to Yosef, and I will charge you less interest', for it is not certain to damage. Yosef might be able to lend his money to another Nochri. This is like preventing someone from using his money (to profit), but he is called a Rasha. The one who ruled that Moshe must give his profit to David erred.