One night, Manuel took Buck from Judge Miller house to sell Buck. Because Manuel gambled a lot money in Chinese lottery and because of that Manuel doesn't have money to support his family anymore. So Manuel once took Buck for a walk and they both arrived to College Park, where Manuel talk with man and sells Buck to man. Man have money to Manuel and this was last time when Buck saw Manuel. This was last time when Manuel appeared.
Manuel was later mentioned several times during book, then Buck remembered that Manuel was that, who sold him.