Manuel had a gambling addiction. He needed money to feed his habit, while at the same time trying to provide for his family. At the time that the story takes place (late 1800's California Gold Rush), there was a strong need for sled dogs for the Yukon. Manuel apparently was aware of this and knowing what sort of potential Buck had, decided that he could sell him for money. While he was correct in his assessment, one could pretty much gather that this was only a short term solution to the larger problem of his financial predicament.
As the story progresses, Buck is sold on a number of other occasions for varying reasons. Getting to the root of all these reasons, the reader will find money as the overriding issue.
One of the challenges I've found in teaching Call of the Wild for so many years, is the advanced vocabulary in London's writing. As a studnent, it's imperative that you "fight against" the tendencey to shut down because of the language. Doing so will enhance your experience with this classic work.