Nedarim, chapter five, mishnah 6. This mishnah opens with a halachah that was already taught in the previous chapter, and its subject is: how one may assist a person who is forbidden by vow from benefiting from his fellow by means of a third party, and where the line runs between a genuine gift and an artificial circumvention of the vow.
The text of the mishnah:
"HaMudar hana'ah mechaveiro v'ein lo mah yochal - nosein l'acher l'shem matanah, v'halah mutar bah" - a person who is forbidden to benefit from his fellow and has nothing to eat, and his fellow wishes to help him: the fellow gives the food to a third party as an outright gift. Once the item belongs to the third party, the one bound by the vow may benefit from it, since this is not benefit from the person who was forbidden to him, but from someone else.
The addition from the words of the Gemara:
The Gemara clarifies that words are missing in the mishnah, and the following must be added: but if the end proves what the intent was at the beginning - that is, when what is apparent at the end reveals the intention behind the giving from the outset. If the one who imposed the vow did not give the gift to the other person outright, but only on condition that he pass it on to the one bound by the vow, the one bound by the vow may not benefit from it. When it is clear to everyone that the purpose of the giving was to enable the one bound by the vow to use the item exclusively, this is not a gift at all, but merely a conduit.
The incident in Beis Choron:
The mishnah brings an incident that supports this principle. There was a man in Beis Choron whose father was forbidden by vow from benefiting from him, for he had vowed not to derive benefit from him, and the son was marrying off his own son and wanted his father to be present at the meal. He said to his friend: the courtyard and the meal are given to you as a gift, and they are before you only so that my father should come and eat with us at the meal.
From here the following exchange developed between them:
The friend said to him: "Im sheli hem - harei hem mukdashin lashamayim" - if they are mine, then they are hereby consecrated to Heaven, meaning that he declares them hekdesh.
The son answered him: "Lo nasati es sheli shetakdishem lashamayim" - I did not give you what is mine so that you should make them hekdesh.
The friend responded: "Lo nasata li es shelach ela shetehei atah v'avicha ochlin v'shosin u'misratzin zeh lazeh, v'he'avon taluy b'rosho" - you did not really give me anything, you only sought to use me as a middleman in order to circumvent the vow, and the sin hangs on your head.
The ruling of the Sages:
When the matter came before the Sages, they said: "Kol matanah she'einah she'im hikdishah tehei mukdeshes - einah matanah" - a gift such that, if the recipient wished to consecrate it, the consecration would not take effect upon it, is not a gift at all. Therefore, in the case of the wedding meal there was no gift in the ordinary sense, and consequently the father was not permitted to benefit from it.
In summary: in this mishnah we learned that giving to a third party permits the one bound by the vow to benefit from the item, but only when the gift is outright. When the end proves what the intent was at the beginning, that the giving was done in order that the item reach the one bound by the vow, this is not a gift but a circumvention of the vow, and as the Sages established: any gift upon which consecration cannot take effect is not a gift.