Reference for Ketubot 4:7
לֹא כָתַב לָהּ כְּתֻבָּה, בְּתוּלָה גּוֹבָה מָאתַיִם, וְאַלְמָנָה מָנֶה, מִפְּנֵי שֶׁהוּא תְנַאי בֵּית דִּין. כָּתַב לָהּ, שָׂדֶה שָׁוֶה מָנֶה תַּחַת מָאתַיִם זוּז, וְלֹא כָתַב לָהּ, כָּל נְכָסִים דְּאִית לִי אַחֲרָאִין לִכְתֻבְּתִיךְ, חַיָּב, שֶׁהוּא תְנַאי בֵּית דִּין:
If he did not write her: "If you are taken captive, I will redeem you and restore you as my wife," or, if he were a Cohein, [who is not permitted to take her back after she was abducted (the wife of a Cohein, who was forced, being forbidden to her husband), in which instance he writes to her:] "I will redeem you and I will return you to your land," he is (nevertheless) liable (for this) [and he must give her her kethubah, even though he must divorce her perforce], for this is a condition of beth-din.