Yevamos, Chapter Two, Mishnah Ten:
"HeChacham she'asar es ha'ishah b'neder al ba'alah - harei zeh lo yisa'enah" - a sage who caused a woman to become forbidden to her husband by means of a vow may not marry her:
A woman took a vow that she would derive no benefit at all from her husband, and because of that vow the couple can no longer live together, so that she must be divorced. They came to a Torah scholar and asked whether he could find an opening to annul the vow, and he was unable to annul it, and on that basis they were divorced. That sage may not marry her. The reason: it looks as though he deliberately refrained from finding a way to permit the vow, so that she would become forbidden to her husband and be divorced, and would thereby become permitted to him.
"Mei'anah b'fanav, o shechaltzah b'fanav - yisa'enah, mipnei shehu beis din" - if she performed mi'un before him, or if she performed chalitzah before him, he may marry her, because he is a court:
This refers to a woman who was married through a rabbinic betrothal, since she was below the age of mitzvos, and who is able to refuse that marriage and nullify it through an act of mi'un; or to a woman who performed chalitzah before that Torah scholar. In both of these cases she is permitted to marry anyone she wishes, and even he himself may marry her, because he is functioning as part of a court.
What is the difference between the two cases?
Annulment of vows: A single recognized expert, a man of standing, can annul the vow on his own. Therefore there is room for suspicion.
Mi'un and chalitzah: These are not performed by an individual but by a court, by a group of more than one person. For that reason there is no suspicion at all, since people do not band together in order to enable him to marry her improperly.
"V'chulan shehayu lahem nashim umeisu - mutaros linasei lahem" - and in all these cases, if they had wives who then died, those women are permitted to marry them:
"V'chulan" - all of them: all those people mentioned in our mishnah and in the previous one, namely the sage who caused the woman to become forbidden to her husband through a vow, and the one who brought a get from overseas and stated "it was written in my presence and signed in my presence." All of these were suspected of having arranged matters so that the woman would be divorced from her husband and become permitted to them. If they had wives at the time of the incident and those wives died, the women in question are permitted to marry them.
True, a man is permitted to marry more than one wife, and one could argue that he wanted to marry her as well, but that was not the accepted practice. And under these circumstances, where only after the death of his first wife does he seek to marry her, we do not believe that a person would calculate so far ahead as to have a woman released from her husband so that she would be available to him should his wife die. We are not concerned with so remote a suspicion.
Two further rulings included in the same principle:
They married others and were then divorced or widowed: Those women who became permitted to remarry through the actions of these men, whether through their testimony or through the vow that brought about the divorce, married another man lawfully, and afterward were divorced from him or widowed. They are permitted to marry those same men, since a person does not make such a far reaching calculation for the future, releasing a woman to marry someone else so that she will ultimately come to him.
They are permitted to their sons and their brothers: Those women are permitted even to the sons and brothers of the men who released them, for we are not concerned that a person would resort to such a scheme and arrange for the woman to be permitted to remarry merely so that his son or his brother could marry her. Only where his own benefit is at stake did they suspect that he might act this way.
In summary: Our mishnah taught that a sage who was asked about a vow and did not annul it may not marry the woman, out of concern that he may have been acting for his own benefit; whereas in the cases of mi'un and chalitzah, which are performed in a court, there is no suspicion and he is permitted to marry her. We further learned that all these suspected individuals are permitted to these women once their own wives have died, or once the women married others and were divorced or widowed, and likewise that the women are permitted to their sons and brothers, since we are not concerned with far reaching calculations or with a benefit that is not his own.