The Alter of Kelm takes note of Efron's change of heart. How could he so quickly go from insisting that Avraham take the cave for free, to accepting a huge sum of silver for it - way above the field's worth? The Torah adds that the money was good money as well. It was money that was acceptable in any country - and Efron grabbed it without further protest. Rashi comments: "He said much, and he didn't even do a little." (of what he promised)
There was once a debate which is famed to have taken place between the Rambam and the philosophers of his day. The philosophers maintained that the nature of an animal can be changed, and it can be transformed into a refined creature. The Rambam maintained that it could not be intrinsically changed. Challenges were made, and the training began. When the day came, a huge gathering was eagerly waiting to witness this historical event. Everyone was astounded to see a cat appear as a waiter, holding a pitcher of wine ready to be poured. Apparently, the philosophers had proven their point and won the argument. The Rambam brought out a little box containing a live mouse, and it was soon scurrying across the floor. Down went the pitcher of wine, and off went the waiter after its prey to the disappointment of all.
Efron was like the cat. He was able to act generously, but the "smell" of a large sum of money overwhelmed him, and out went Mr. Generous. "Maybe I'll be generous tomorrow." Imagine if Efron had known that his deeds would be forever read by generations, and lessons of how not to be would be learned from him. What would he have done differently? As we "write the story" of our own lives we would do well to learn from Efron.