Jerusalem Talmud Ketubot 6.2

משנה: הַפּוֹסֵק מָעוֹת לַחֲתָנוֹ וּמֵת חֲתָנוֹ אָמְרוּ חֲכָמִים יָכוֹל הוּא שֶׁיֹּאמַר לְאָחִיךָ הָיִיתִי רוֹצֶה לִיתֵּן וְלָךְ אֵי אֶפְשִׁי לִיתֵּן.

MISHNAH: If somebody decided on money for his son-in-law and the son-in-law died, the Sages said that he can say, I was ready to give to your brother, but it is impossible for me to give to you.

הלכה: הַפּוֹסֵק מָעוֹת לַחֲתָנוֹ וּמֵת חֲתָנוֹ כול׳. וְלֹא דְבָרִים שֶׁהֵן קוֹנִין בָּאֲמִירָה הֵן. תַּנֵּי בַּר קַפָּרָא. פּוֹסֵק לְשֵׁם כְּתוּבָּה עַל מְנָת לבנים. וְכֵינִי. הַפּוֹסֵק מְעוֹת לְבִתּוֹ קְטַנָּה כּוֹפִין אוֹתוֹ לִיתֵּן. הָא לִגְדוֹלָה לֹא. אָמַר רִבִּי אָבוּן. כּוֹפִין אוֹתוֹ לִיתֵּן גֵּט.

HALAKHAH: “If somebody decided on money for his son-in-law and the son-in-law died,” etc. Are these not the things that are acquired by speech? Bar Qappara stated: He promised the dowry on condition there be a marriage. And it is so: If somebody decided on money for his underage daughter, one forces him to give. But not for the adult one? Rebbi Abun said, one forces [the groom] to give a bill of divorce.