Baba Kama 58a - Dina D'malchusa

In R' Asher Weiss's weekly shiur on parshas mishpatim he quotes 2 fundamental approaches for the source of dina d'malchusa. The Ran and Rashbah take the approach that the land is owned by the king so he can implement what he wants. Whereas the Rashbam (baba basra 54b) and the Rambam (Hilchos gezeila 5:18) take the approach of "heskem b'nei hamedina", meaning that the inhabitants of a particular area accept certain rules and principles on themselves (there are also more approaches such as rashi in gittin who implies that it is based on the fact that goyim are commanded on dinim, which gives them authority to impose their dinim even on the jews). Based on the approach of "heskem b'nei hamedina", the rambam makes a distinction between an actual king and a warlord or mafia, since those groups may be powerful but have not been accepted by the population. This approach also seems to be the basis of the Ramah quoted in Tur (c.m. 369) who says that tax increases that are being imposed against popular opinion would not be justified using the rule of dina d'malchusa.

Tosafos about mavriach ari, at the very end speaks about Jews who were forced to escape from their land to avoid taxes that they could not afford. The land would then be taken by the government and sold off to others. The R"I holds that if the government sold it to another jew, the sale is not binding since they had no right to sell it in the first place (but the original owner would have to compensate for benefits he received). Tosafos explains that this is not dina d'malchusa, rather it is gezeila. Tosafos doesn't consider it gezeila due to the lack of authority such as a warlord or mafia type, rather tosafos says - ואם יש שר שבא לשנות את הדין ולעשות דין לעצמו אין זה דינא דמלכותא שהרי זה הדין אין הגון כלל

Tosafos seems to understand that the concept of dina d'malchuas is limited to what people consider justice, not what people perceive as theft. Therefore, even a government who technically has the power to impose dina d'malchusa, they can only do what is considered by most people to be "fair". This would also be based on the approach of the Rambam and Rashbam that the concept of dina d'malchusa is based on consensus of it's citizens. 

Baba Kama 52a - Locking the Door: Chazaka?

Tosafos quotes the Rashbam who explains that נעל וגדר כל שהוא בנכסי הגר does not refer to merely locking the door, because that would not qualify as a chazka to acquire the house, rather it would be מבריח ארי by preventing a damage which doesn't qualify as a kinyan. Tosafos disagrees and proves from other places that locking the door would constitute a kinyan. Tosafos explains that locking the door is more than just preventing damage, because it demonstrates ownership by not allowing anyone else to enter.

Both Tosafos and the Rashbam agree that preventing damage would not qualify as a kinyan, but they argue whether locking the door is just preventing damage or demonstrating ownership. Why doesn't preventing damage qualify as a kinyan? The SM"A (choshen mishpat 192:10) writes:

שסילוק ההיזק אינו קונה, זה לשון הגמ' משום דהו"ל מבריח ארי מכנסי חבירו, פירוש רשב"ם ז"ל הא למאי זה דומה למשיב אבידה דכל ישראל מצווין להציל ממון חבריהם מן ההיזק

The Rashbam holds that since 'mavriach ari' is an obligation on every jew to save the property of another, it doesn't demonstrate ownership and cannot qualify as a kinyan. R' Akiva Eiger points out that based on this rationale, in the estate of a ger who died, since his estate is hefker, the logic of protecting another jews property no longer applies, therefore it should qualify as a kinyan. The Rashbam himself as quoted by Tosafos clearly holds that even in the estate of a ger, protecting the estate does not constitute an acquisition. The Pischei Teshuva quotes from the Ramban that even in the estate of a ger where there isn't any mitzvah, one cannot acquire by merely protecting the property from damage since it is just an act of preventing loss and not an act of a kinyan.

Nedarim 41b - Providing Refuah for a Choleh

The Ran explains that a doctor is allowed to provide refuah to a choleh, even though there is a neder forbidding him to provide benefit to the choleh, since: 1. it is a mizvah. 2. he is using the medicine of the choleh himself, and therefore is not literally giving him anything. This is even permitted when there is someone else available to heal the choleh, because it could be that this doctor will be the one who will be zocheh to be successful so we consider it as if there is no other doctor available. But, a veterinarian cannot heal the animal of the mudar since he is benefiting him. However, the Ran says that if there is no other vet to heal the animal, the madir can do it because it is a mitzvah of hashavas aveida. This implies that if there are other veterinarians around, there is not a mitzvah of hashavas aveida. The rationale is that if there are other people to do it, since this individual is forbidden with a neder to provide benefit, the mitzvah is incumbent upon the others who are not forbidden with this neder, and once others are able and available to do it, it is no longer an "aveida" so there is no mitzvah on him. But if there is no one else available to do it, then there is a mitzvah of hashavas aveida and even the individual who is assur b'neder can do return the object or heal the animal.
The Ya'avetz understands that there isn't any machlokes between the Ran and Rosh, since the Rosh is talking when there is another vet available to do it and therefore not a mitzvah of hashavas aveida (although the language of the Rosh sounds more that there is no mitzvah to heal someones animal). R' Akiva Eiger asks, that by returning an aveida the sevara of mitzvah is not the primary heter (although the nimukei yosef uses the sevara of mitzvah to explain why the tircha is allowed), rather the main heter is that it is saving the object from a loss and mavriach ari is not hana'ah. This is only applicable to a lost object, but healing his animal is tantamount to feeding it which is considered a hana'ah and not just saving him from a loss, so it should be assur even if there is no one else to do it? Perhaps the Ran considered the healing of an animal to also be saving him from a loss, since the disease is threatening the animal.

Nedarim 33b - Mavriach Ari

We already discussed this somewhat in kesubos 107 http://hearos.blogspot.com/search?q=mavriach but there is more to talk about.

The Ran assumes that we need the sevara of mavriach ari for all 3 cases listed in the mishna. The gemara discusses paying a debt for someone who is assur to receive hana'ah from you. The heter is either based on the opinion of Chanan who says that when reuven pays shimon's debt on the condition that shimon will owe him money, shimon does not owe money to reuven since he never requested the loan, therefore when he does it on the condition of mechila it is not considered benefiting shimon - that is called mavriach ari, meaning he simply prevents shimon from suffering a loss. The other possibility is that we are speaking about a case where shimon doesn't actually owe anything since he stipulated with the lender not to collect, so when reuven pays the "debt" for him, shimon is not receiving any real gain.

The Ran understands that by being paying his shekel we can also use the concept of mavriach ari according to Chanan. However, it is slightly more complicated because aside from the concern of getting the gizbar (who is like the lender) off shimon's back, reuven is buying shimon a share in korbanos. Therefore the Ran has to say that even if shimon does not pay, he is still entitled to a share in the korbanos (as rashi writes in kesubos, not like tosafos there).

Being that the Ran holds that mavriach ari applies to the paying the shekel as well, he must hold that mavriach ari applies even to a situation where the damage is definite. This is to the exclusion of Tosafos who writes that mavriach ari only applies when reuven saves shimon from the fear of a lion but not from an actual lion. Tosafos holds that if the damage is definite then reuven cannot protect shimon from it, since it qualifies as providing him benefit. Tosafos can only say this because they understand the case of paying the shekel is speaking according to everyone where shimon already payed and it was lost, but if shimon has not yet payed then reuven cannot pay for him because shimon has no way out of the situation, it is a definite loss and therefore is not called mavriach ari. The Ran clearly disagrees with Tosafos and holds that even when the damage is definite such as paying his shekel that was never payed for, we can apply mavriach ari.

It is not so clear how the Ran will answer Tosafos question from the case in Baba Metzia where a shepherd saves the herd from a lion attack and we require the owner to pay the shepherd and do not consider it mavriach ari. Why would we consider the case of paying the shekel to be mavriach ari? See first answer of Tosafos B.M. 31b for a possible answer, but it needs explanation.

Kesubos 108a - Benefiting from One who Is Mudar Ha'na'ah

The gemara quotes the mishna in nedarim 33a that if reuven made a neder not to receive any benefit from shimon, shimon may pay his shekel, pay his debt and also return his lost object. Regarding the heter of paying his debt the gemara explains that it is either based on chanan or it is not a regular type of debt rather a debt that does not have to be payed back. Regarding the heter to pay his shekel which presumably buyes reuven a portion in the korbanos tzibur, Rashi maintains that even without giving the money reuven would have a portion in the korbanos, therefore shimon is not benefiting him at all. This implies that if reuven's portion in the korbanos would be pending on his shekel, then shimon would not be able to give the shekel for him.

Regarding shimon returning reuven's lost object, the gemara says in nedarim 33b "he is giving back to him his own object", meaning that reuven is the owner of the object anyway so he is not receiving anything from shimon. The Ran 33b seems to be bothered that shimon is still helping out reuven and returning the object that would otherwise be lost, why is that not a benefit being given to reuven? He answers that it qualifies as mavriach ari - meaning chasing away potential damage to his object. However, this doesn't seem to be sufficient. Perhaps shimon picking up the object to protect it from damage may be "mavriach ari", but the searching for reuven and tircha invested into the actual return of the object should still be considered a benefit that he is providing for reuven?