I believe that the answer is requires a better understanding of what keddushin really is. Keddushin is not a real acquisition. Contrary to what many may wish to believe, a husband does not own his wife. When a man performs an act of a kinyan on a woman it creates another type of transaction, marriage. Keddushin is a state that two parties enter into by means of a kinyan. However, the woman does not belong to her husband. The state of keddushin does require certain obligations from each of the bound parties.
Therefore the sale of Me’aras Hamachpela is indeed an ideal source for the fact that one may use money to off set the status of keddushin. For in both scenarios an actual acquisition was not accomplished by the kinyan.
This also answers another question. The Gemara in Keddushin 26a says that the source in the Torah that one may use kesef to acquire land is from a pasuk in Yirmeyahu. Tosafos’s there asks why the Gemara did not cite the pasuk in this week’s parsha as a source for this halacha. Based on what we have explained, the Gemara could not have cited the pasuk by Avraham Avinu’s acquiring of Me’aras Hamachpella as a source for all real estate acquisitions, for Avraham had already owned the land.