Bava Kamma, Chapter 10, Mishnah 5. The Mishnah moves on to deal with taking land that does not belong to a person. The foundation of these matters is the rule that land cannot be stolen - from a halachic perspective, it is impossible to steal land. This rule is derived from an exposition of a verse, and its logic is that land is like a coordinate on a map, and always remains in its place.
We have already seen that the Torah makes a fundamental distinction between possession and ownership: a watch belonging to Reuven that is seized by Shimon is still Reuven's watch, only it is found in the hands of another. Even if Shimon takes it with him to the ends of the earth, the ownership does not change, and the mitzvah of returning it rests upon him. With land, the matter is even simpler: the land stands in its place, it is impossible to grasp it and carry it away, and it remains where it has always been.
Therefore, even if Shimon took Reuven's land - pointing a gun at his head and ordering him to leave the property - and even if he sold it to a third party, and the third to a fourth, and so on a thousand times, the land remains in the ownership of its original owners. The exchange of possessors changes nothing, and even the owners' despair of getting it back is ineffective. The land is his, and this is the Torah's view.
What, then, is 'stealing land'?
Nevertheless, the Torah uses the expression "one who steals his fellow's field". The intention is not stealing in its technical sense, since this is not possible with land according to Torah law, but rather seizing possession. This can occur in two ways:
Openly: Shimon points a gun at Reuven's head and orders him to leave the field immediately. Reuven departs and Shimon enters in his place, making him a squatter.
Secretly: Shimon arrives at night and moves the boundary line - the markers or the fence - and stealthily takes over Reuven's property.
In both cases, the land remains technically in Reuven's possession, but since Shimon is using it unlawfully, it can be treated as robbery to a certain extent.
Returning stolen land:
Our Mishnah deals with the robber's obligation of returning it. The simple rule is that since land cannot be stolen, it is enough for the robber to leave the land and say to the owner: Here is your land before you. However, the Mishnah focuses on the question of what the law is when the owner can no longer receive it into his hands, and brings two such scenarios.
The first case - "unetaluha masikin":
Shimon stole Reuven's field, and afterward "unetaluha masikin" - extortionists took it. The authorities authorized people to confiscate the land, and the squatter sitting there is removed from it. In this regard, the Mishnah distinguishes:
"Im makas medinah hi" - if it is a regional disaster. When the confiscation affects everyone and is not specifically related to the robber, "omer lo harei shelcha lefanecha" - he may say to him, "Here is yours before you". Shimon returns to Reuven his property as is, and with that the matter is concluded. And if his desire is to repent fully out of true regret for the entire deed - that is an added virtue, but there is no obligation upon him beyond this, since land cannot be stolen.
"Ve'im machamas hagazlan" - but if it is due to the robber. When the confiscation came only because Shimon was sitting there, and they would never have taken this land had it been held by Reuven, "chayav leha'amid lo sadeh acher" - he is obligated to provide him with another field. Since Reuven cannot get his field back, as the confiscator is sitting in it as a squatter, and this would not have happened had Shimon not entered and caused the trouble - Shimon is obligated to provide him with another field or its value as compensation.
By Torah law, this is not the rule, since land cannot be stolen, and Shimon can say: Your field is right here before you, take it. However, the Sages penalized and punished him for being a robber, and since Reuven's loss came about because of him - they obligated him to compensate him by Rabbinic law.
The Gemara's deduction from the phrase "machamas hagazlan":
The Gemara asks: The language of the Mishnah seems redundant. Once it stated "im makas medinah hi" - if it is a regional disaster, it would have been enough to say "and if not" - meaning that if it is not a regional disaster, the robber must compensate. Why did it specifically state "machamas hagazlan" - because of the robber?
The Gemara answers that this deduction comes to teach an additional case that the Mishnah is including: The king turns to Reuven and informs him that he is confiscating all his properties, and demands to see a list of all his assets on his desk. Reuven submits the list, and adds to it - whether intentionally or by mistake, it makes no difference - the assets of his enemy, Shimon. The result is that the king confiscates not only Reuven's properties but also Shimon's, and it turns out that Shimon lost out solely because of Reuven's action. Reuven caused him damage that is very close to direct damage, even though he was not the one who actually carried it out.
This is what is called 'garmi', as opposed to 'garma':
Garma: Indirect damage, for which one is not liable by human law in the earthly courts, but only by the laws of Heaven.
Garmi: Damage that is close to direct causation, as in our case, where it is clear that the loss occurred only because one person handed over the details of his fellow's assets - and for this he is liable.
The exact definition of the difference between garma and garmi is a dispute in the Shulchan Aruch, and this is not the place to resolve it. Either way, we learn from our Mishnah that someone who hands over his fellow's property to the authorities by including it in a list - has acted unlawfully, and he must pay his fellow compensation.
The second case - the flooding of the river:
In contrast to confiscation by the authorities, if after Shimon removed Reuven from the field and invaded it, a river overflowed its banks and flooded the entire field until it was destroyed - "omer lo harei shelcha lefanecha" - he says to him, your property is before you. Since this is purely an act of God, an event that would have occurred whether Reuven was in the field or Shimon, Shimon can say: I am leaving the field, take yours, and with that the matter is concluded - and he is exempt. He has no additional obligations by human law, since he returned the field and no longer holds onto it; and even though Reuven receives a useless field, ultimately this would have been his fate regardless, and therefore Shimon is not penalized even by Rabbinic law.