What does Juan Tomas try to caution or worn his brother as they walk toward the city?

Juan Tomas warns Kino and Juana to get the best price for the pearl, and tells him how their ancestors got an agent to sell their pearls, but this agent ran off with the pearls. Kino had heard the story told as a warning of punishment against those who try to leave their station. Kino and Juana, followed by neighbors, reach the offices of the pearl buyers.