It’s not kofin oso if he was mechayev himself in it (we’re back to square one on this issue).
And if you say that he wasn’t really mechayev himself in it because he never expected to actually have to pay it, and therefore it’s an asmachta, how is that different from 200 zuz which he also never expected to have to pay? If in the kesubah he’s mechayev himself to pay 200 if he divorces her, and in the prenup he’s mechayev himself to pay X if he doesn’t divorce her, which one is the asmachta?
Of course, all this may only be relevant if the husband doesn’t want to divorce her. But if he does want to divorce her, and is holding out for money or custody or to be me’agen her, there may not be such a thing as a get me’useh at all:
Igros Moshe EH 3:44
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Chazon Ish EH 99:2
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