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Beis Shamai holds that a person may make a Neder to thieves or tax collectors only if he is prompted, and one may never make a Shevu'ah.
Beis Hillel holds that a person may make both a Neder and Shevu'ah to the thief even if he is not prompted. (1)
Shmuel says that Dina d'Malchusa Dina. (2)
Dina d'Malchusa Dina does not obligate a person to pay a tax collector who does not collect a fixed amount or a self appointed tax collector.
When someone makes a Neder to a tax collector that he is prohibiting the fruit of the world if his food if not Terumah, he should say in his heart that it is only for one day. (3)
Beis Shamai says that it is only permitted to make a Neder to a tax collector if he was prompted to make the Neder.
Beis Hillel says that one may swear even on items that he was not prompted to swear about.
Rebbi Ashi says that Beis Shamai holds that a Chacham may not release someone from a Shevu'ah, but Beis Hillel argues. (4)
If someone says that these trees shall be a Korban if they are not cut down today, they are Hekdesh and they may be redeemed. (5)
If someone says that these trees shall be a Korban until they are cut down they may not be redeemed. (6)
Even if there was a very strong wind that day and he was positive that the trees would blow down if they do not blow down they are Hekdesh.
If someone says that these trees shall be a Korban until they are cut down Bar Pada says that after they are cut down they need to be redeemed. (7)
Ula holds that once the trees are cut down they are no longer Hekdesh and they do not need to be redeemed.
A BIT MORE
1. He may make a Neder or a Shevu'ah that all of the fruit of the world shall be prohibited if his food is not Terumah or if his possessions do not belong to the King.
2. Therefore if a tax collector bought the right to collect taxes from the king it is forbidden to evade paying the taxes.
3. Even though normally Devarim sheba'Lev (unspoken words) are not effective, but in a case of Ones they are effective, since it is obvious to all that he is making the Neder under duress.
4. According to Beis Shamai a Chacham may only release someone from a Neder and not from a Shevu'ah.
5. If the day goes by without them being cut down they are Hekdesh and they may be redeemed.
6. If they are redeemed they immediately go back to being Hekdesh because he said that they should be a Korban until they are cut down.
7. Even though he said that they should be a Korban until they are cut down however Bar Pada holds that Hekdesh cannot disappear on its own and therefore they need to be redeemed.
DINA D'MALCHUSA DINA
The Ran says in the name of Tosfos that Dina d'Malchusa Dina only applies in Chutz la'Aretz because the King can say, "If you do not want to keep my laws then get out of my land," however in Eretz Yisrael the King cannot threaten to throw us out of our land because every person in Klal Yisrael has a portion in Eretz Yisrael.
If someone vows that these trees shall be a Korban if they are not cut down today, or this Talis shall be a Korban if it does not get burned today if the day passes by without them being cut down or without the Talis being burned they are Hekdesh and they may be redeemed like any other Hekdesh and after their redemption one may benefit from them. But if someone says that these trees shall be a Korban until they are cut down he may not redeem them because whenever he redeems them they will go back to being Hekdesh. Once they are cut down it is not necessary to redeem them and it is permitted to benefit from them immediately. (Rambam Hilchos Me'ilah 4:11)
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