According to Mishna 7:5 of Masechet Gittin, if a man gives a woman a divorce on condition that she give him 200 zuz, she is divorced and must give it to him. But this seems to make the ketubah useless in the case of divorce, because a man could just always divorce his wife on the condition that she pay him more than the value of her ketubah. This seems impossible to me. What am I missing?
Note that this is a question because a woman can be divorced against her will in halacha (at least before the takanah of Rabbeinu Gershom).