The Gemara asks what is the difference between the Migu that Rebbi Yehoshua agreed to (when he says "This field used to belong to your father and I bought it from him" Migu he could have said "It always belonged to me"), and the Migu that he did not hold of (she says "I was raped after betrothal", and you owe me the whole Kesuvah, Migu that she could say that she lost her Besulim through being Mukas Etz after the betrothal).

Tosfos proved on 13a (DH Rav Asi) that Rav Asi holds that where there is a "Migu Gamur" even Rebbi Yehoshua agrees that we say Migu. A "Migu Gamur" is where the claim a person could have made in Beis Din would have been a successful one. This is as opposed to the Migu that Rebbi Yehoshua does not agree to which is where the claim a person can make is a better claim (for example it is less suspicious to the Beis Din or it incurs the one who makes the claim less embarrassment).

The Maharsha asks: since this part of the Gemara is going according to Rav Asi (the Gemara previously explained the Mishnah according to Ze'iri), and Rav Asi holds that Rebbi Yehoshua agrees to Migu Gamur, how can the Gemara ask what is the difference between the two cases of Migu? The case of the field is a "Migu Gamur" because if the person would say that this field was always mine he would be believed by Beis Din, however the case of the woman claiming the Kesuvah is not a Migu Gamur because the alternative claim would not be a successful one.

The Maharsha explains that this is in fact the answer of the Gemara, as explained by Tosfos (16a DH Hasam). The Gemara means that in the case of Kesuvah where we are aware of the fact that she was not a Besulah at the time of the Chupah and she is unable to deny this fact, she would not be believed even with the alternative claim (that she lost her Besulim through being Mukas Etz after the betrothal). However in the case of the field the alternative claim would be believed.

Dov Freedman