Kesubos, Chapter 7, Mishnah 1. The Mishnah deals with a man who takes a vow forbidding his wife to derive benefit from him, and with the question of how long such a situation may be allowed to stand.
"HaMadir es ishto miliheinos lo":
A man who takes a vow that his wife may not derive any benefit from him, meaning that he will not support her. At first glance such a vow should not be able to take effect, since a husband is obligated to his wife and is required by law to provide her sustenance, and he has no power to cancel that obligation with a vow.
The Gemara explains that the Mishnah is speaking of a case where he said to her, "Let your handiwork go toward your sustenance" - that is, you are not obligated to give me your earnings as is customary, and in exchange I am not obligated to provide your sustenance, and your work will supply your needs. In such an arrangement he may indeed vow that she will not benefit from him, and she supports herself from her own labor.
The Mishnah, however, is speaking of a case where her work suffices only for her basic sustenance, but not for her smaller and more marginal needs - needs for which the husband remains responsible. It is for this situation that the Mishnah instructs how to proceed.
"Ad sheloshim yom yaamid parnes":
For up to thirty days the husband may arrange for another person to see to her needs. But the Gemara points out: since he vowed that she should not benefit from him, he cannot appoint an agent or instruct a particular person to care for her, because that person would be acting as his agent, and she would in effect be benefiting from him.
The Gemara therefore explains the Mishnah as referring to a case where he does not appoint anyone directly, but rather states in general terms, "Whoever attends to her will not lose out." There is no instruction to any particular person, that person is not acting as his agent, and the husband may afterward pay him for what he did for his wife, since this is not direct benefit.
"V'im yoser mikan - yotzi v'yitein kesubah":
This leniency applies for thirty days only. If thirty days passed and he did not annul his vow and found no way to release it, and his wife no longer wishes to be supported by that third party - "yotzi v'yitein kesubah" - he must divorce her and pay her kesubah. The reason, as the Gemara explains: after thirty days the matter becomes publicly known, and this is a source of shame for the woman.
The dispute between Rav and Shmuel in defining the case:
Rav: The measure of thirty days applies only when the husband specified that his vow would remain in force for thirty days. But if he vowed without specifying and did not set a time limit, he must divorce her immediately, since from the outset this is a situation she cannot be expected to endure.
Shmuel: Even when he vowed without specifying, we wait thirty days, in case he finds a way to have his vow released during that time and will not need to divorce her. If thirty days passed and he found no release - he must divorce her and pay her kesubah.
The view of Rabbi Yehudah regarding the time limit:
The Mishnah brings the view of Rabbi Yehudah, who distinguishes between the wife of a Yisrael and the wife of a kohen:
"B'Yisrael - chodesh echad yekayeim, u'shnayim - yotzi v'yitein kesubah" - in the case of a Yisrael, who is not a kohen, he may support her through that third party for one month; in the second month he must give her a get and pay her kesubah.
"U'v'chohenes - shnayim yekayeim, u'shloshah - yotzi v'yitein kesubah" - in the case of the wife of a kohen, where once he divorces her there is no way back and they can never remarry, he may maintain her for two months; in the third month he must divorce her and pay her kesubah.
Thus the wife of a kohen is given a longer period of time than the wife of a Yisrael.
In summary: In this Mishnah we learned the law of one who vows that his wife may not benefit from him, which is explained as referring to a man who told her "Let your handiwork go toward your sustenance" while her work does not suffice for her smaller needs; the permission to set up a provider for up to thirty days, not through direct appointment but through a general statement that whoever attends to her will not lose out; the obligation to divorce her and pay the kesubah after thirty days because of the shame of the matter becoming public; the dispute between Rav and Shmuel regarding one who vowed without specifying; and the time periods set by Rabbi Yehudah - one month for the wife of a Yisrael and two months for the wife of a kohen.