Yerushalmi Kiddushin 3:11 ירושלמי קידושין ג׳:י״א

MISHNAH: “I am preliminarily married to your daughter,” but she says, you are only preliminarily married to me; he is forbidden the relatives of the young woman²²⁴Since he claims marriage with her daughter, he is forbidden the relatives he could not marry had the claim been true (cf. Mishnah Yebamot4:7). While all relatives of the mother’s are also relatives of the daughter’s, the lists of forbidden unions are different in the two cases., and the young woman is permitted his relatives; he is permitted the relatives of the old woman, and the old woman is forbidden his relatives.

HALAKHAH: “I am preliminarily married to your daughter,” etc. Rav Ḥuna said, one induces him to give a bill of divorce, and one forces him to pay a fine²²⁶It is not clear from this text to which woman the bill of divorce has to be given. If sense is to be made of the text, one has to read it in the light of the Babli, 65a, where Samuel states that if a woman claims to be preliminarily married to a man, which claim he denies, one has to try to make him give her a bill of divorce to free the woman for marriage to another. In the case of the Mishnah, the bill of divorce would be for the mother; the daughter is not mentioned in the discussion. Rav, the teacher of R. Ḥuna, states that if he gave the bill of divorce on his own initiative, one can force him to pay the divorce settlement required by the law of ketubah.. Think of it, if he was a Fellow and knew that he would be obligated for a bill of divorce and a fine; if he went and married her sister, would one force the divorce of the certain because of the doubtful²²⁷Rav Ḥuna’s statement is rejected as impractical. Assuming that the man either is reasonably well trained in the law or has access to legal advice, he can avoid being badgered into offering a bill of divorce and then forced to pay a hefty sum by threatening to marry the woman’s sister. Since the Mishnah permits him to marry the sister, one cannot ask him to give a divorce to his sister-in-law since by that act he would prohibit his prospective wife to himself. It is in the interest of the woman desiring the bill of divorce to make it clear that she has no monetary claims. (Explanation of D. Fraenckel.)
The last sentence cannot be read as declarative: “one forces the divorce of the certain because of the doubtful”, meaning that the man would be barred from marrying the sister because of the unproven claim of the woman, since this would contradict both the Mishnah which permits the woman’s relatives to the man, and the general principle that in marriage matters only those statements count which can be backed up by the testimony of two witnesses. The case cannot be compared to Yebamot 3:4 where a man is forced to divorce his (rabbinically) preliminarily married wife and give ḥalîṣah to her sister.
“Fellow” is used here not in the technical sense of a person observing the laws of ritual purity (cf. Introduction to Tractate Demay), but somewhat learned in the law.?