On gimmel umud beis the gemara says that "Es Biti" is telling us that the father is koneh the rights to kedushin if so why do we need "Ain Kesef"?
Dear Michael,
"Es Biti" only tells us that the father is in control of marrying his daughter but it doesn't say how! (Maybe only Shtar or Bi'ah works.) We need a source for money!
However, Tosfos (DH Hashta) in Kesuvos 46b asks this in a slightly different manner. He says we know money works from "Kichah" from Efron, and then "Es Biti" adds that it goes to the father, and thus "Ein Kesef" is extra, which Tosfos does not answer.
The Shitah Mekubetzes there (DH Hashta) answers that even with the source of Efron and Kichah, perhaps that is only if she is a Bogeres and does Kidushin herself and the money is needed for a Kinyan, but the father can marry her off without money. For this we need "Ein Kesef" to tell us that Kidushin done by the father also needs money.
All the best,
Reuven Weiner