Resources for Nedarim 28
1. The משנה says that you can make a נדר to a מוכס indicating that your food is תרומה so he will not take it. The גמרא in the מסקנא understands this to be talking about a case where he said his נדר סתמא and thought to himself only today. Otherwise it is a valid נדר. The ריטב"א back on דף כ"ז asks why this isn’t considered נדרי אונסין. He answers that נדרי אונסין is when you never meant to make a נדר in a case where a river would block you from coming. However, by our משנה you did mean to make a נדר as it was only money on the line. (My understanding is that the ריטב"א is coming from כעין the concept mentioned in גיטין of תליוהו וזבין זביני זביני which means if you are forced to make a sale, a person will still make the sale with full דעת as long as he gets paid. Similarly here, if a person is forced to make a נדר then he makes the נדר with full דעת as it is a necessary tradeoff to keep his money.)
2. The גמרא says in the name of שמואל that דינא דמלכותא דינא. There is a fundamental מחלוקת as to where this concept comes from. The ר"ן here says that its based on the fact that the king owns the land. Therefore, he has the right to kick you out if you don’t accept his rules, just like any homeowner can kick you out of his house if you don’t accept his rules. Consequently, this law would apply in every country other than ארץ ישראל because every Jew has a portion of ארץ ישראל and the king has no ownership of it. The רשב"ם in בבא בתרא דף נ"ד ע"ב ד"ה והאמר שמואל disagrees and says it has nothing to do with ownership. Rather, everyone implicitly agrees to the laws of the land (as they need society to function). Therefore, if you take money from your friend based on secular law it is not considered stealing. The שו"ת חתם סופר in חו"מ סימן מ"ד narrows the מחלוקת considerably. He says that even the ר"ן agrees to the רשב"ם by normal business transactions. (In fact the ר"ן himself in דרשות הר"ן in his דרשה about מלכות says that the מצוה of appointing a king (שום תשים עליך מלך) is not a בדיעבד. Rather the king is needed to create secular laws to help ensure society can function. For example, if Jewish society is not at a high level ח"ו and murder is rampant, society won’t function if you need two עדים and התראה all the time to stop a murderer. That’s where the king comes in and creates his own laws. The ר"ן is obviously talking about a Jewish King in Israel and yet he clearly has authority to make laws.) Rather, the ר"ן only argues with the רשב"ם by taxes because people never agree to taxes. It is only because the king owns the land that he has the right to collect taxes. The בית שמואל in אבן העזר סימן כ"ח ס"ק ג says that even if according to secular law a person is not קונה an item with יאוש ושינוי רשות, the person is קונה on a דאורייתא level and therefore his קידושין done with such an item would be חל מדאורייתא. This implies that דינא דמלכותא דינא is just a דין דרבנן. The אבני מילואים there in ס"ק ב asks why this would only be a דין דרבנן? The מנחת שלמה here in נדרים suggests that the בית שמואל understood like the רשב"ם that דינא דמלכותא דינא is based on everyone’s implicit הסכמה. However, it’s מסתברא that this would not be stronger than a type of הפקר ב"ד הפקר where we would say that קנינים דרבנן don’t help for a דאורייתא. Similarly here, דינא דמלכותא דינא would have the power of a תקנת ב"ד but not be relevant to דאורייתא קנינים.
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