Yevamos, Chapter 15, Mishnah 3. This Mishnah deals with a further question: even though a woman is believed when she says that her husband died overseas, may she collect her kesubah on the strength of that testimony? This is a different question - a monetary one.
The dispute between Beis Shammai and Beis Hillel:
Beis Shammai: She may marry on the strength of her own testimony, and she also collects her kesubah.
Beis Hillel: "Tinasei" - she may marry on the strength of her testimony, but she does not collect her kesubah. When it comes to money and property she is not believed until she brings witnesses.
The claim of Beis Shammai - a kal vachomer from a forbidden relationship to money:
Beis Shammai said to Beis Hillel: "Harei hitartem ervah chamurah" - you permitted her in a matter of forbidden relationships, the most severe area of Torah law. If so, all the more so should a monetary matter be permitted, which is lighter and not nearly as severe.
Beis Hillel answered them: we find that the brothers do not enter the inheritance on the strength of her word - the brothers are not permitted to take possession of their inheritance and benefit from it based on her statement alone, since for that purpose we do not treat the husband as dead.
Beis Shammai's proof from the wording of the kesubah document:
Beis Shammai responded again: from the text of the kesubah itself, from the body of the written document, we can learn that she does collect her kesubah, for the husband writes to her in the kesubah that if she marries another man she will take what he wrote for her.
It emerges that the two matters are explicitly bound up with one another: the husband obligates himself in the kesubah and in its payment on the basis of her being able to marry another man. And since the Sages were prepared to permit her to marry another man, it follows automatically, from the very definition of the terms, that she is entitled to collect her kesubah.
In conclusion, Beis Hillel retracted and ruled in accordance with the words of Beis Shammai, that she collects her kesubah as well on the strength of her testimony that her husband died.
In summary: in this Mishnah, Beis Shammai and Beis Hillel disagree as to whether a woman's testimony about her husband's death is effective for collecting the kesubah as well. Beis Shammai argued with a kal vachomer from the severe matter of forbidden relationships to the lighter matter of money, and Beis Hillel answered from the law of the brothers, who do not enter the inheritance on the strength of her word. Once Beis Shammai brought proof from the wording of the kesubah document, in which the husband makes the obligation of the kesubah dependent on her marrying another man, Beis Hillel retracted and agreed that she collects her kesubah.