The גמרא discuss the concept of יש מעילה בקונמות. There is a lot of discussion as to how far and literally we take this. The רא"ש in the משנה on דף ל"ג seems to understand that the item you made אסור with a נדר is literally owned by הקדש. Therefore when you make someone אסור בהנאה from yourself and you do something for them that they would normally pay for (like returning a lost item), the money must be given to הקדש since you cant be מפסיד ממון הקדש. That sounds like it is real ממון הקדש. The ר"ן in ד"ה יש מעילה mentions that if you violate your נדר and get הנאה, you must bring an actual קרבן מעילה. However, the מנחת שלמה points out that the ר"ןdoes not mention a need to pay a קרן וחומש להקדש. Furthermore, the ר"ן also says that in the case mentioned above (returning an item to a person who cant get הנאה from you where people normally pay for the service), you don’t need to give the money to הקדש. The מנחת שלמה says that this seems to point to the ר"ן holding that it is not real הקדש and the חיוב קרבן is just because of the חטא. He adds, however, that we never find that kind of hybrid concept.

The ר"ן brings a ספק in the ירושלמי as to what the הלכה is once you use something you were אסור to use because of a נדר: do we say it is יוצא לחולין like מעילה does and now the נדר is gone or that it stays אסור? He leaves it as a ספק.

The משנה למלך in הלכות מעליה פּרק ד הל׳ ט has many other ספקות in this regard. First, if you violate your נדרdo you always get מלקות twice, once for לא יחל דברו and once for מעילה? As to the type of נדר, does it matter if your נדר made an item אסור on everyone or just one person? Perhaps the discussion of יש מעילה בקונמות is only if you made it similar to הקדש by making it אסור on everyone, but if you just made it אסור on a few people then there would be no מעילה (the case of our משנה that talks about returning a lost item is presumably talking about someone who didn’t אסור themselves on the whole world in which case the רא"ש who says there is מעליה would not work). Another similar question is if you only make it אסור באכילה and not בהנאה would it be subject to מעילה if it is not similar to הקדש in that way? Also, what if you weren’t מתפּיס anything in your נדר (meaning you just said “this should be אסור on me” and didn’t say “this should be אסור on my like a קרבן”)? Perhaps it isn’t like מעילה if you didn’t declare it to be a קרבן. He says this may be תלוי on another fundamental מחלוקת from the beginning of the מסכתא: does every נדר need התפסה? If you hold every נדר needs התפסה and of you don’t say “like a קרבן” then your נדר only works מטעם יד where we fill in the words for you as if you said it, then you will always have an issue of מעילה since we always treat it as if you said "כקרבן", but if you hold you don’t need התפסה then perhaps יש מעילה בקרבנות doesn’t apply if you aren’t מתפּיסו בקרבן. What is interesting is that theרמב"ם paskens that you don’t need התפסה, and is then very מדוייק in הלכות מעילה פּרק ד הל׳ טwhen he says “יש מעילה בנדרים. כיצד האומר ככר זו עלי קרבן”. You see like the משנה למלך suggested that the only time we would say יש מעילה בקונמות is if you said explicitly עלי כקרבן. The מנחת חינוך inמצוה קכ"ז אות י"ד says that if you say something should be אסור to you like נדרי שלמים or some other קדשים קלים then there certainly isn’t מעילה since there isn’t even מעילה by real קדשים קלים. The מנחת שלמה points out something interesting: by a typical case where you say כקרבן we say there is מעילה, even though you might have meant קדשים קלים. This must be because we always assume לחומרא by a נדר and assume you meant קדשי קדשים. However, if this is so, how can the ר"ן be מסופּק if it is יוצא ‌חולין? We should be חושש you meant קדשים קלים לחומרא and there never was מעליה and it wasn’t יוצא לחולין!? The questions are numerous, and the exact גדר doesn’t seem so clear.

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