Kesubos, Chapter 4, Mishnah 9. This mishnah does not deal with one of the clauses of the kesubah, but rather with a detail of the husband's obligation to redeem his wife.
Redeeming a Captive Wife:
The mishnah states: "nishbeis - chayav lifdosah" - if she was taken captive, he is obligated to redeem her. As we have mentioned, the husband bears the obligation to redeem a wife who has been taken captive. And if he said: "harei gitah u'chesubasah, tifdeh es atzmah" - here is her get and her kesubah, let her redeem herself, meaning: I hereby divorce her and pay her her kesubah, and with the money of the kesubah let her redeem herself - "eino rashai" - he is not permitted to do so. He does not fulfill his obligation in this way, since his obligation to redeem her preceded this.
Medical Expenses:
In contrast, the mishnah continues: if she became ill, "chayav leraposah" - he is obligated to heal her. The husband must bear the costs of her medical care until she recovers, and this obligation is considered part of his obligation to provide her with sustenance, that is, the supplying of food and support. However, if he said: "harei gitech u'chesubasech, terapei atzmah" - here is your get and your kesubah, and from the money I have paid you, heal yourself - "rashai" - he is permitted, and he may indeed do so.
What is the difference between the two obligations?
The obligation of medical care: This obligation is part of her sustenance, part of the payment for her food and her care, and it is in force as long as they are married to one another. Once they are divorced and he has paid her her kesubah, she takes upon herself the responsibility for her own sustenance and her own medical care. Therefore the husband may act in this way, since this is part of the divorce process.
The obligation of redemption: This obligation was never paid. The Sages instituted the obligation of redemption as compensation for the fact that the husband receives the produce of the melog property that the wife brings with her into the marriage, and this compensation has not yet been given. For this reason he cannot say to her: here you are, take care of yourself. The obligation stands in place and must be paid, and therefore he has no option of saying this.
We should note that the Rambam, in citing this halachah, writes that the husband is obligated to pay for her medical care, and if he said to her: "harei chesubasech, v'shalmi bishvil atzmech" - here is your kesubah, and pay for yourself - "shom'in" - we listen to him and allow him to do so, but "ein ruach chachamim nochah heimenu" - the spirit of the Sages is not pleased with him, from the standpoint of "derech eretz": it is not proper to act this way, for this is not the manner in which a person ought to conduct himself.