Lake….batei din aren’t mesader kidushin; that claim falls flat on its face.
The rema says if someone makes a knas on himself during kidushin that he’ll pay X amount if he divorces his wife, it’s a probable get mi’usa. Some rabbis, including a couple of actual poskim, say that the prenup is different, because it requires mezonos instead of a knas….i really don’t get what the difference is.