Nedarim, Chapter 9, Mishnah 4. In this chapter we have already seen the concern that a person will not tell the truth when asked whether he really would not have taken the vow - out of concern for his own honor, and according to Rabbi Eliezer, also out of concern for his parents' honor. And all agree that a person does not tell the truth regarding the question of whether he would have refrained from the vow on account of the covenant of the Holy One, Blessed is He. Our Mishnah, however, teaches that when we open an opening for a person's vow by saying that had he known that his vow would bring him to transgress certain mitzvos, we believe him when he tells us whether he would have taken the vow or not.
The words of the Mishnah:
"Ve'od amar Rabbi Meir" - in addition to what he said in the previous Mishnah, Rabbi Meir also said: "posechin lo mah shekasuv baTorah" - we open an opening for him from what is written in the Torah. "Ve'omer lo: ilu hayisa yodei'a" - and we say to him: had you known that through your vow you would come to transgress one of the following mitzvos:
"Al tikom" - the prohibition of taking revenge, that you will come to take revenge against your fellow because of your vow.
"Ve'al lo sitor" - the prohibition of bearing a grudge.
"Velo sisna es achicha bilvavecha" - the prohibition of hating one's fellow in one's heart.
"Ve'ahavta lerei'acha kamocha" - the mitzvah of loving your fellow as yourself.
"Vechei achicha imach" - the mitzvah of supporting your brother, for because of your vow he is liable to become poor, "ve'ein atah yachol lefarneso" - and you will not be able to support him and fulfill this mitzvah with him.
"Ve'amar: ilu hayisi yodei'a shekein, lo hayisi noder" - and when the one who vowed responds that had he known this, or any one of the matters listed above, he would not have taken the vow.
"Harei zeh mutar" - the vow is permitted, and this is a valid annulment of vows on the basis of that opening.
In summary: in this Mishnah Rabbi Meir teaches that we open an opening for the one who vowed from what is written in the Torah - from the prohibitions of taking revenge, bearing a grudge and hating in one's heart, and from the mitzvos of "ve'ahavta lerei'acha kamocha" and "vechei achicha imach" - and when he responds that had he known he would not have taken the vow, he is believed, and the vow is permitted.