why does the gm bring a proof for kesef
firstly the mishna says it
secondly the proof is from a tranaction which gives the buyer different kind of rights
chaim , is
1. The Gemara knows that Kidushin can be with Kesef, as it says in the Mishnah, but is asking from where in the Torah did the Mishnah learn this Halachah.
2. The Gemara says we can learn that Kidushin can be with money from the fact that Kidushin is referred to as "Kichah" in the Torah, and we find elsewhere (buying fields) that the Torah calls a transaction done with money "Kichah". From this we learn that when the Torah says the word Kichah in connection to Kidushin it is referring to a Kinyan that is performed with money. The fact that the Kinyan of buying a field is different to the Kinyan of Kidushin does not matter as the Gemara is just inferring from the term used how the Kinyan is made.
Dov Freedman