Resources for Nedarim 34
The גמרא says that if a person says to a הפקר piece of bread ככר זו הקדש, then when he goes to pick it up with intention to eat it he is מעל. The question that many of the ראשונים discuss is how did he make it הקדש to start with if he never picked it up? The ר"ן says that the case is he is within four אמות of the item and he was קונה with aתקנה דרבנן called קנין ד אמות. This is a special type of קנין which תוספות inבבא מציעא דף י ע"א ד"ה ארבע says only works for מציאות so that people shouldn’t fight about it and by גיטין to prevent עיגונא.
תוספות here as well as the ר׳ אליעזר ממי"ץ give another possibility. They say that the case is that he was more than four אמות away but he is the closest person to the object so it is considered בידו to pick it up. Therefore, once he in fact picks it up הקדש is קונה. (and since he picked it up to eat it at the same time he is also עובר מעילה.) The תוספות רי"͏ד says a different פּשט which is an incredible חידוש. He says that הקדש has a special rule of אמירתו לגבוה כמסירתו להדיוט. Therefore, all he has to say is that its הקדש and since in the meantime its no one else’s it works to be קונה for הקדש. Interestingly, the מנחת שלמה understands our תוספות to be saying similar to the תוספות רי"͏ד. תוספות seems to hold that there is no קונה per se until you pick something up. However, the מנחת שלמה argues that that isn’t an issue. One could argue that one could be מקדש anything, even if you don’t own it, except that there is a דרשה that says ואיש כי יקדיש את ביתו, מי ביתו ברשותו אף כל ברשותו. However, since this is בידו to pick up its called ברשותו and you can be מקדיש it immediately even though an official קנין has not occurred. The קצות החושן in סימן ר"ד ס"ק ג seems to have understood that תוספות and the ר׳ אליעזר ממי"ץ meant that an actual קנין had been made just by being closest to it, and רע"אin ב"מ דף מ"ט ע"א disagrees.
The end of the גמרא says that if you say “my bread is אסור on you”, you cant give it to the person as a gift as you clearly meant to forbid that. However, the גמרא says that perhaps you intended that if you gave it away to him it would be allowed, and you just meant to say that if you invite him as a guest to your table that he is אסור to have it. The ר"ן explains that if this is true, the reason the guest can’t have it is because at that point its still owned by the בעל הבית. This sounds like its saying that when you invite people over to a meal, they don’t own the food. The קובץ שיעורים in פּסחים דף כ"ט אות קל"א brings this ר"ן as a question on the רמ"א. The רמ"א in אבן העזר סימן כ"ח סעף י"ז says that a guest who picks up a piece of food and gives it to a lady as קידושין is מקודשת. רב אלחנן זצ"ל asks that this should be against the ר"ן who says a guest is not the owner of the food he is offered. He answers that the בעל is not מכוון להקנות to his guest. Rather, the guest is not מכוון לקנות until he eats it. However, if he picked it up to give a lady for קידושין, clearly he was trying to be זוכה in it.
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