Responsa for Yevamot 60:14

ואי נמי זימנין דאתא אחר ומקדש לה לדידה קדושי ודאי וכיון דאסר לה מר לצרה לייבומי אמרי דקמא קדושין ודבתרא לאו קדושין

<b><i>GEMARA</i></b>. In the case of divorce, however, It is not stated IT WAS UNCERTAIN WHETHER IT FELL NEARER TO HIM OR NEARER TO HER; what is the reason?<span class="x" onmousemove="('comment',' Why should not even halizah on the part of the rival, be required in such a case? ');">³²</span> -Rabbah replied: This woman<span class="x" onmousemove="('comment',' The rival. ');">³³</span> is in a state of permissibility to all men;<span class="x" onmousemove="('comment',' Lit., 'to the market', i.e., the public. The rival of a forbidden relative, not being subject to levirate marriage or halizah. is permitted to marry any one she desires. ');">³⁴</span> would you forbid her [marriage] because of a doubt?<span class="x" onmousemove="('comment',' The possibility that the forbidden relative's divorce was valid. ');">³⁵</span> You must not forbid her because of a doubt!<span class="x" onmousemove="('comment',' The doubt here being whether the forbidden relative was divorced at all. In the three cases of divorce mentioned in our Mishnah, however, the prohibition Is not due to doubtful divorce but to a defect or an irregularity in the document itself. ');">³⁶</span> Said Abaye to him: If so, let us also in the matter of betrothal say: This woman<span class="x" onmousemove="('comment',' The rival. ');">³⁷</span> is in a state of permissibility to the levir;<span class="x" onmousemove="('comment',' Had her husband died childless before he married the forbidden relative. ');">³⁸</span> would you forbid her<span class="x" onmousemove="('comment',' To be taken in levirate marriage. ');">³⁹</span> because of a doubt? You must not forbid her because of a doubt! — There<span class="x" onmousemove="('comment',' The case of doubtful betrothal. ');">⁴⁰</span> [it leads] to a restriction.<span class="x" onmousemove="('comment',' The prohibition to marry the levir. ');">⁴¹</span> But it is a restriction which may lead to a relaxation! For, sometimes, he would betroth her sister<span class="x" onmousemove="('comment',' The sister of the one whose betrothal was doubtful. ');">⁴²</span> by betrothal that was not uncertain, or it might occur that another man would betroth her also by a betrothal that was not uncertain and, as the Master has forbidden her rival to be taken in levirate marriage. it would be assumed that the betrothal of the first<span class="x" onmousemove="('comment',' Since her rival is forbidden. ');">⁴³</span> was valid and that that of the latter was not!<span class="x" onmousemove="('comment',' Because, in the first case, he betrothed his wife's sister; and, in the second, he betrothed a married woman. In the latter case, the betrothal being regarded as invalid, the woman might illegally marry another man. In the former case, should he die without issue, his maternal brother might illegally marry her, believing her never to have been the wife of his brother. ');">⁴⁴</span>

Teshuvot Maharam