Nedarim, Chapter 8, Mishnah 7. This mishnah deals with vows that were made conditionally or within a particular context, and with the question of how to assess the true intention of the person making the vow.
One who vows in order to compel his fellow to accept a gift:
"Ha'omer lachavero: konam she'eini nehneh lach, im ein atah ba venotel levincha kor echad shel chittim ushtei chaviyot shel yayin" - a person says to his fellow: I vow that I will derive no benefit from you at all, unless you come and take for your son a gift of a kor of wheat (a large measure) and two barrels of wine. In other words, he wants the other man to accept the gift, and if he does not accept it, the vow will take effect.
"Harei zeh yachol lehafer et nidro shelo al pi chacham" - a vow of this sort can be nullified even without a sage. Normally a Torah scholar or a court is required in order to release a vow, but here that is not necessary.
Why? Because the fellow, the very person from whom the one who vowed forbade himself benefit, can say to him: "Kelum amarta ela mipnei kevodi - zehu kevodi". That is: you said all this only in order to honor me by giving a gift to my son, yet my honor lies precisely in not accepting it. Since the vow was made on the condition that the other man be honored through it, and he is indeed honored through it, just in a different way, the vow never takes effect at all.
One who vows that his fellow will not benefit from him:
"Vechen ha'omer lachavero: konam she'atah nehneh li, im ein atah ba venoten livni kor echad shel chittim ushtei chaviyot shel yayin" - here the one who vows forbids his fellow to derive benefit from him, unless he gives his son a kor of wheat and two barrels of wine. In this case the one who vows wishes to be the recipient of the gift, and he forbids himself to his fellow as long as it has not been given.
Rabbi Yirmiyah says: "Asur ad sheyiten" - as long as the fellow has not given the gift, he may not derive benefit from the one who vowed.
And the Sages say: "Af zeh yachol lehafer et nidro shelo al pi chacham, veyomar lo: harei ani ke'ilu kibalti" - here too the vow can be released without a sage and without a court, since the one who vowed says that as far as he is concerned it is as though he had already received the gift, and therefore the vow does not take effect.
Vows whose intent concerns marriage:
"Hayu mesarvin bo lisa et bat achoto, ve'amar: konam shehi nehneit li le'olam" - they pressed a man to marry his sister's daughter, and he vowed that she would never derive benefit from him. It is worth noting that there is value and a mitzvah in marrying one's sister's daughter, which is considered a fulfillment of the verse "and from your own flesh do not hide yourself," when it is a suitable match.
"Vechen hamegaresh et ishto ve'amar: konam ishti nehneit li le'olam" - one who divorces his wife and vows that she will never derive benefit from him.
"Harei elu mutarot lehanot lo, shelo nitkaven zeh ela leshem ishut" - in both of these cases the women are permitted to derive benefit from him, since it emerges from the context that his intention was to vow only that he would not be married to them.
In the case of the divorce, where this is not a situation of vengefulness, the husband is stating with certainty that he will not remain married to her; and likewise in the case of his sister's daughter, the man is making clear that he does not wish to marry her. He does not intend to rule out every possible benefit, to the point that he could not hand her a napkin or that they could not pass each other an item of food. He means to say that he refuses a marital bond, and therefore it is clear from the context that his words are not meant in the broad sense they seem to convey.
One who vows not to dine at his fellow's home:
"Hamesarev bachavero sheyochal etzlo, ve'amar: konam leveitcha she'eini nichnas, tipat tzonen she'eini to'em lach - mutar likanes leveito velishtot mimenu tzonen" - a person who refuses his fellow's invitation to dine with him, and vows that he will not enter his house and will not even taste a drop of cold water from him, is permitted to enter his house and to drink a little cold water there.
The reason for this: it is clear from the context that his intention was only to vow that he would not eat a full meal at his home. As for a small matter, it is evident from the context that he did not intend to include it in his vow, but only an actual meal.
In summary: in this mishnah we learned of four types of vows that do not take effect according to their plain meaning: one who vows in order that his fellow take a gift for his son, and the fellow answers him "this is my honor"; one who vows in order that his fellow give a gift to his son, where according to the Sages he says "it is as though I had received it" - and in both of these he can nullify his vow without a sage; one who vows against his sister's daughter or his divorced wife, whose intention concerns marriage alone; and one who vows not to dine at his fellow's home, who intended only an actual meal. The principle that emerges from them all is one: we assess the intention of the one who vows from the context, rather than clinging to the plain sense of his words.