QUESTION: The Mishnah states that if a man finds his wife to be a Be'ulah and claims that the marriage was an error ("Mekach Ta'us"), and the woman claims that she was raped after she became betrothed to him (and thus the marriage was not a Mekach Ta'us), Raban Gamliel and Rebbi Eliezer rule that she is believed, and Rebbi Yehoshua rules that she is not believed. The Gemara (11b) cites an opinion that when Rebbi Yehoshua says that she is not believed (and thus the marriage was a Mekach Ta'us), she does not lose her entire Kesuvah but only 100 Zuz. Why does she not lose the entire Kesuvah? There is a Safek that she willfully committed Z'nus, and thus she should not receive anything!
(a) TOSFOS (9b, DH Iy) explains that she gets part of the Kesuvah because of a Sfek Sfeika: there is a doubt that perhaps she had Z'nus before the Erusin, and even if she was unfaithful after the Erusin, perhaps it was b'Ones (against her will). Since there is a Sfek Sfeika in her favor to collect 100 Zuz for her Kesuvah, she may collect. Regarding the other hundred, however, there is a Sfek Sfeika against her: perhaps she had Z'nus willfully, and even if it was b'Ones, perhaps it was before the Erusin.
The PNEI YEHOSHUA (9b) asks why should she be able to collect money with a Sfek Sfeika? There is a rule that "Ein Holchin b'Mamon Achar ha'Rov" -- we do not follow a "Rov" when it comes to monetary matters. A Sfek Sfeika should not be any better than a Rov, as a Sfek Sfeika works by creating a situation of a majority (Rov) of possibilities in one's favor. The Pnei Yehoshua explains that Tosfos means that she may collect money through a Sfek Sfeika only because she also has a "Ta'anas Bari" -- she is certain of the truth of her claim. (See REBBI AKIVA EIGER and SHEV SHEMAITSA 1:23.)
(b) The SHEV SHEMAITSA (1:23) cites the MAHARI BEN LEV who says that a Sfek Sfeika indeed cannot be used to collect money. He gives a different explanation for why she collects 100 Zuz of the Kesuvah. He says that the reason she collects the money is because when it comes to the Isur, she will be permitted to her husband because of the Sfek Sfeika. If she is permitted to him, then her entire Kesuvah cannot be taken away from her; since she is allowed to remain married to him, he must assume some responsibility towards her, and thus he may not revoke her entire Kesuvah. The Shev Shemaitsa suggests that this is the intention of Tosfos. The Pnei Yehoshua says the same logic but uses it as an argument against Tosfos.
(c) The SHEV SHEMAITSA himself (1:24) writes that perhaps a Sfek Sfeika may be used to collect money only in the case of a Kesuvah. The HAGAHOS MORDECHAI (in Yevamos) writes that since the Kesuvah is already written in a contract, Beis Din may follow a Rov to collect the money, since it is "k'Man d'Gabuy Dami" -- it is as though it has already been collected since the woman is holding the contract of the Kesuvah. Therefore, the money of the Kesuvah may be collected through a "Rov" since she is not considered as though she is taking money out of someone else's possession. He adds that since the Kesuvah is an act of Beis Din, it is considered as though the woman is already holding the money, and thus she is not taking it away from him with a Rov, but rather the Rov merely allows her to hold on to the money that is already in her possession.