Nedarim, Chapter Three, Mishnah One
The first four mishnayoth of this chapter deal with vows that use valid language but are nevertheless not valid because the person did not really intend for his vow to be valid. The first category is one who vowed in order to give himself or someone else more incentive to do something. This might be like today if someone says I bet you a million dollars that I will do a certain thing. If it doesnt happen, no one expects the person to actually pay a million dollars. People say these things in order to give themselves more incentive.
1) Four types of vows the Sages have invalidated: Vows of incentive, vows of exaggeration, vows in error, and vows [broken] under pressure.
2) Vows of incentive how so?
a) If one was selling an article and said, Konam that I will not reduce below a sela; and the other replied, Konam that I will not add above a shekelboth of them want [a price] of three denarii.
3) Rabbi Eliezer ben Jacob says: Also one who wishes to subject his friend to a vow to eat with him, may say: Every vow which I may make in the future shall be void, providing that he remembers this at the time of the vow.
Section one: The first section of the mishnah lists those vows which are not valid. The mishnah and the following mishnayoth will now bring examples of each.
Section two: In this scenario both the buyer and the seller make vows that they will not budge in their prices. One says he will not go above a shekel (two denarii) and the other says he wont go below a sela (four denarii). Since their intention was for a price of three denarii, they may agree to that price.
Section three: Rabbi Eliezer ben Jacob says that one who wants to get his friend to eat at his place, and vows that if his friend does not do so all of his food will be forbidden to him, should say beforehand that all of his vows that he takes in the future shall be invalid, and then he need not keep his vow. However, when he makes the vow, he must remember that he made the original stipulation that his vows would not be valid.
The above explanation of Rabbi Eliezer ben Jacobs statement is according to the words in our mishnah, where it seems as if he is stating one halakhah. However, the Talmud explains that Rabbi Eliezer ben Jacob actually says two things. First of all, a vow taken to get another person to eat at ones place, is a vow of incentive, and therefore need not be kept. Second, a person may state that all future vows will be invalid, and as long as he remembers this statement when he vows in the future, his vows will be invalid. By the way, the latter statement is one of the sources of the practice to nullify future vows at Kol Nidre on Yom Kippur.