Commentary for Ketubot 6:4
פָּסְקָה לְהַכְנִיס לוֹ כְסָפִים, סֶלַע כֶּסֶף נַעֲשֶׂה שִׁשָּׁה דִינָרִים. הֶחָתָן מְקַבֵּל עָלָיו עֲשָׂרָה דִינָרִין לַקֻּפָּה, לְכָל מָנֶה וּמָנֶה. רַבָּן שְׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, הַכֹּל כְּמִנְהַג הַמְּדִינָה:
If she committed herself to bring in silver to him, [actual dinars with which he can trade immediately], a sela of silver [which is four dinars] becomes six dinars. [It is written into the kethubah as six dinars, an addition of a third. The tanna hereby apprises us that with actual dinars, whether they be many (such as the "thousand dinars" [6:2] or few (such as the "sela" here), they are always written into the kethubah at a third more, for they are immediately negotiable. And with things that require assessment, such as clothing and jewelry, whether they be many (as in "and relative to the assessment" [6:2], which refers to the thousand zuz mentioned above) or few (as in "for an assessment of a manah" [6:3]), she gives thirty-one sela and a dinar. In the kethubah, we always write a fifth less than the assessment of the bridal chamber. In this connection, it makes no difference whether it was assessed first and then came to be written into the kethubah, in which instance we must deduct in the kethubah a fifth of the assessment of the bridal chamber (as we learned [6:3]: "Whatever the groom writes, etc.,") or whether he writes the kethubah first, in which instance the assessment must be a fifth more than what is written in the kethubah (as we learned [Ibid.]: "And for a four manah assessment, she gives five manah.")] The groom takes it upon himself to give to the "fund" twenty dinars for every manah [i.e., to give her ten zuz for every manah that she brings him, with which to buy spices and perfumes. Our Mishnah does not specify whether every week, every month, or every year.] R. Shimon b. Gamliel said: "All is in accordance with the custom of the land.