What does Kino offer the doctor as payment for the doctor's treatment?

In chapter 1, Coyotito is bitten by a poisonous scorpion and Juana suggests that they take their son to the doctor in town. When Kino and Juana arrive at the doctor's residence with their ill son, the doctor's native servant answers the door. After Kino explains why Coyotito needs the doctor's attention, the servant relays the message to the doctor, who is uninterested in helping the "little Indians." Kino then offers to pay the doctor with eight small, misshapen seed pearls, which are of little value. The doctor reveals his racist, callous nature by refusing to treat Coyotito.
After Kino finds the largest pearl ever to be discovered in the region, he gains attention from his community, and the word spreads throughout town. The doctor ends up visiting Coyotito in Kino's village, and it seems as if Coyotito is recovering from the poisonous sting. However, the greedy doctor gives Coyotito a mysterious capsule filled with powder, which ends up making Coyotito sick. After the doctor leaves, he returns promptly in an hour and gives Coyotito three drops of ammonia, which calms the infant. The doctor then feigns ignorance about Kino's massive pearl and asks him how he plans on paying for his services. Kino responds by telling the doctor that he will pay him back once he sells the pearl. The doctor then offers to put Kino's pearl in his safe before he leaves the village.