Yerushalmi Ketubot 4:11 ירושלמי כתובות ד׳:י״א

MISHNAH: If he did not write her “the male children you will have from me shall inherit the amount of your ketubah in addition to their part with their brothers²³⁷If the wife should die before the husband and the husband inherits from her then, if the husband dies, the sons of his several wives can first claim their mother’s ketubah (the ≥200 zuz from the husband and all the dowry noted in the document) from the estate; only the remainder is divided between the sons (with an eventual first-born son taking two parts.)”; he is obligated, for this is a stipulation enforced by the court.

HALAKHAH: “If he did not write her “the male children,” etc. That is what Rebbi Immi said, that a man should be disposed to give generously to his daughter²³⁸If he knows that the dowry will remain the possession of his grandchildren, he will be more inclined to provide his daughter with a dowry. In the Babli, 52b, the explanation is ascribed to R. Joḥanan and the recommended amount is 10% of the father’s property.
The same statememt is also in Halakhah 10:2..

²³⁹A similar, but more tersely and clearly formulated text is in Baba Batra 8:6. There, we have stated²⁴⁰Mishnah Baba Batra 8:6. The biblical law of inheritance (Num. 27:8–11) states that inheritances should be distributed to the next of kin; it is implied that all persons of the same degree of relationship shall be treated equally (except for the rule of the firstborn. Deut. 21:15–17). In Mishnah 8:5, the anonymous majority states that a person may make a will in which he distributes his property unevenly. The will will be valid if formulated in the language of gifts, invalid in the language of inheritance. In Mishnah 8:6, R. Joḥanan ben Beroqa states that even the language of gifts is acceptable if and only if the recipient is a legal heir directly in line; e. g., one son among many, but not a daughter in the presence of sons since daughters inherit only if there are no sons. If there are no sons, R. Joḥanan ben Beroqa will accept unequal distribution of the estate among the daughters. The Mishnah here is formulated following Ben Beroqa since it accepts unequal distribution of the estate even without writing (Babli Baba Batra 131a).: “Rebbi Joḥanan ben Beroqa says, if he speaks about somebody who is a legal heir, his statements are valid, but about somebody who is not a legal heir, his statements are invalid.” Rebbi Joḥanan said, Ben Beroqa spoke only about a son among sons or a (daughter)²⁴¹This was the first (correct) word written by the scribe who then (incorrectly) crossed it out and replaced it by “son” to produce a meaningless sentence. son among daughters, not about a daughter and brother or a brother among daughters. Rebbi Yasa in the name of Rebbi Joḥanan: There was a case and Rebbi decided following Rebbi Joḥanan ben Beroqa. Rebbi Ze‘ira, Rebbi Yasa in the name of Rebbi Joḥanan: Rebbi asked Nathan the Babylonian, why did they say that practice follows Rebbi Joḥanan ben Beroqa? Rebbi Abba, Rebbi Ḥiyya in the name of Rebbi Joḥanan: What is the reason? He asked Nathan the Babylonian, why did the Sages say that practice follows Rebbi Joḥanan ben Beroqa²⁴²Since the Mishnah in Ketubot is anonymous, it represents practice and it follows R. Joḥanan ben Beroqa. The Babli, Baba Batra 131a, presents the question in slightly different form and ends up by denying its validity. The Babli holds (seemingly against the Yerushalmi) that property rights based on a ketubah stipulation are strictly weaker than those mentioned in the ketubah contract in that they cannot be asserted against third parties.? Are you not the one who stated “shall inherit”? (He said to him, you stated “shall take”; he came to him)²⁴³The fragments of sentences in this parenthesis make no sense; they belong to nearby texts and the scribe seems to have been unsure about the order of the sentences. It is best to leave the parenthesis unexplained.. He said to him, there is no “shall inherit” but “shall take”²⁴⁴It is not quite clear whether this is Rebbi’s or R. Nathan the Babylonian’s statement. It changes the language of the Mishnah, that the woman’s sons “shall take her ketubah”, making the transaction one of a gift valid from the time of the marriage. Therefore, the rules of inheritance are irrelevant and there is no relation between the Mishnah here and Ben Beroqa’s statement about wills. (Everybody agrees that by a gift “valid now but executed only after my death”, a person can freely dispose of his assets without regard to biblical laws of inheritance.). When he came to his father, he said to him²⁴⁵When Rebbi came to Rabban Simeon ben Gamliel, the latter affirmed the text of the Mishnah as the correct one.: you disadvantaged Nathan the Bablylonian; there is no “shall take” but “shall inherit”. Rebbi Yose ben Rebbi Abun said, this is what he told him: I made an error in what I taught you; there is no “shall take” but “shall inherit”. Rebbi Ze‘ira said, this is one of the leniencies of the ketubah stipulations. In general, a person can transfer rights only by a document; here even verbally. In general, a person cannot transfer rights to somebody not yet existing; here to the non-existing²⁴⁶At the date of the marriage, no sons are yet born. For everybody except R. Meïr, the unborn cannot be the recipient of a gift (other than through a guardian); the same argument is in the Babli, loc. cit.. Rebbi Jeremiah said before Rebbi Ze‘ira: I am wondering how the rabbis can compare ketubah to inheritance, for inheritance is a biblical law but ketubah is from their words²⁴⁷Ketubah is purely a rabbinic institution (or better, a generally followed practice in post-exilic Judaism, inspired by but not derived from the biblical “bridal gift”.) This is the opinion of Rabban Simeon ben Gamliel, cf. the last paragraph of this tractate.
Since the Mishnah text is Rabban Simeon ben Gamliel’s, the question is relevant.; how can one argue this? Even if you say that ketubah is a biblical law, do they not agree that the stipulations of the ketubah are from their words²⁴⁸Therefore, there is no connection between the ketubah stipulations and the laws of inheritance.?