In terms of the argument between Hai Gaon and the Rosh [R Asher] [In Rav Shach Laws of Marriage 19: law 19.] it seems to me that Hai Gaon must have been thinking about the fact that in the Gemara the law is Lots of land נכסים מרבים the boys inherit and they support the girls. A small amount of land all the profits [rent or fruit in farm land] goes to support the girls but if they sell, then the girls lose. The ambiguous thing here is what happens if they sell when there is lots of land?  The Rosh thought obviously it is the same thing. The girls lose. But Hai Gaon may have thought that the girls would get the proceeds of the sell even in the law of the Gemara--before we even get to the decree of the geonim that the Ketubah can be collected from movable property.

I mean the Rosh has a point that proceeds of a sell of an object is not usually thought to be in place of an object unless we would be talking about maasar Sheni or some kind of חלות קדושה


What I think here is that Hai Gaon must have thought that when the Gemara says if the boys sell when there is not much property that the sell is anyway valid that that does not exclude the girls from getting the proceeds of the sell--even from teh law of the Gemara itself