Nedarim Chapter 9, Mishnah 5. The Mishnah teaches that a person can be given an opening to have his vow annulled based on his obligation to pay his wife's kesubah.
The case before us:
A man vowed that he would derive no benefit from his wife and that she would derive no benefit from him, and her kesubah was four hundred dinar - quite a substantial sum. They came before Rabbi Akiva, and he told him: since you have made this vow, you cannot live with her, and therefore you must divorce her and pay her the kesubah.
The husband replied: Rabbi, my father left us an inheritance of only eight hundred dinar; my brother took four hundred and I took four hundred. "Lo dayah" - is it not enough for her that she take two hundred and I take two hundred, and we split it between us? Why must I pay her four hundred?
Rabbi Akiva said to him: "Afilu atah mocher se'ar roshecha" - even if you have to sell the hair of your head, you are obligated to pay her the kesubah.
Explanation of the Mishnah's wording:
In truth, even if he were to sell the hair of his head, the hair is not mortgaged to the payment of the kesubah, since a kesubah is collected only from land - from real estate - and not from movable property. Rather, the intent is this: even if you should be reduced to poverty because you were forced to give up your land, to the point that in order to survive you would have to sell your hair - even in such a state you are obligated to give her the kesubah.
The opening for annulling the vow:
When the husband heard this, he said: "Had I known this, I never would have made the vow in the first place." On the strength of this opening Rabbi Akiva ruled that she is permitted to him and that he need not uphold his vow, since the vow can be annulled - and consequently there is no need for a divorce and no obligation to pay the kesubah.
In summary: In this Mishnah we learned that the obligation to pay the kesubah itself serves as an opening for annulling a vow. The husband who vowed that he would derive no benefit from his wife and that she would derive none from him was required to divorce her and pay her four hundred dinar, and when he claimed that he could not afford it, Rabbi Akiva told him that even in utter straits he is obligated in her kesubah. Once it became clear that he had not made the vow with that in mind, his vow was annulled and the obligation to divorce and to pay fell away.