We begin our study of the first Mishnah in the fifth chapter of Maseches Kesubos, which deals with two matters: the possibility of adding to the fixed sum of the kesubah, and the possibility of reducing it.
Adding to the sum of the kesubah:
The Mishnah opens: "Af al pi she'amru besulah govah masayim ve'almanah maneh" - the Sages fixed a set sum for the kesubah: a virgin collects two hundred zuz and a widow collects one hundred zuz. Nevertheless, "im ratzah lehosif, afilu meah maneh - yosif". If the husband wishes to add to the fixed sum, in either of these cases, and even a hundred maneh, which is an enormous sum, he is entitled to do so.
The Gemara clarifies that we should not say that the Sages set a uniform sum so as not to embarrass someone who cannot afford more. This is not one of the cases in which we are concerned about that, and therefore there is nothing improper about wealthy people increasing the sum. This addition is what is called 'tosefes kesubah'.
The Mishnah continues: "Nis'almenah o nisgarshah, bein min ha'eirusin bein min hanisu'in - govah es hakol". A woman whose kesubah included an addition, and whose husband died or divorced her, collects the full sum including the addition. We do not say that she collects only the basic sum, even in a case where there was only betrothal, where one might have thought that the husband is not obligated in the addition.
Rabbi Elazar ben Azaryah disagrees with the first Tanna and distinguishes between the two situations:
From the marriage: "govah es hakol" - if he brought her into his home and they began to live together as husband and wife, and afterwards she was divorced or widowed, she collects the full sum including the addition.
From the betrothal: "besulah govah masayim ve'almanah maneh" - if there was only kiddushin, a virgin collects two hundred and a widow one hundred, and the addition is not included in what she collects.
The Mishnah gives the reasoning behind his view: "shelo kasav lah ela al menas lechonsah" - the husband obligated himself in the addition only out of the expectation that he would indeed bring her into his home and that they would live together as husband and wife. Since that did not come about, he never intended to obligate himself in the additional sum.
Forgoing part of the sum of the kesubah:
The second part of the Mishnah discusses whether a woman may forgo part of the kesubah and exempt her husband from the full sum. Rabbi Yehudah permits this, and brings two cases:
A virgin: "im kosev libesulah shtar shel masayim" - the husband writes her a kesubah for the required sum, "vehi koseves: hiskabalti mimcha maneh", meaning that she confirms in a receipt that she has already received one hundred zuz out of the two hundred.
A widow: he writes her a maneh, as the required sum, "vehi koseves: hiskabalti mimcha chamishim zuz" - an admission that she has already collected half of the sum.
In practice she received nothing, but she decided to forgo the money and exempt him from payment, and the way she does so is by claiming that she has already received part of the sum. In Rabbi Yehudah's opinion it is permitted to act this way.
Rabbi Meir disagrees and says: "Kol hapoches libesulah mimasayim vela'almanah mimaneh - harei zo be'ilas zenus". Anyone who gives a virgin less than two hundred or a widow less than a maneh, the relationship between them is considered promiscuous, as if the physical bond took place outside the boundaries of marriage.
The Gemara clarifies that according to Rabbi Meir, that receipt she wrote, or a condition she stipulated to forgo half of the kesubah, is of no effect at all. The condition is void, and in practice the woman will be able to collect the full sum.
Even so, the very writing of such a receipt or the making of such a condition causes her to feel insecure regarding the sum due to her. The entire purpose of the kesubah is that the woman should feel secure in her husband's financial commitment, and damaging that sense of security is a deficiency in the marriage itself. For that reason the relations between them are considered as though they took place outside the boundaries of marital intimacy, like one who cohabits with a harlot, and therefore they are forbidden according to Rabbi Meir.
In summary: in this Mishnah we learned that a husband is entitled to add to the fixed sum of the kesubah as much as he wishes. According to the first Tanna the woman collects everything, including the addition, whether from betrothal or from marriage; while according to Rabbi Elazar ben Azaryah the addition is collected only from marriage, since he wrote it for her only on condition that he would bring her into his home. Regarding reducing the sum, Rabbi Yehudah permits a woman to forgo part of it by writing a receipt, while Rabbi Meir holds that the condition is void, and since the woman's sense of security is damaged, this is considered be'ilas zenus.