He uses reverse psychology to to trick favour in revealing the location of the pearl.

From the text:

The doctor shrugged, and his wet eyes never left Kino's eyes. He knew the pearl would be buried in the house, and he thought Kino might look toward the place where it was buried. "It would be a shame to have it stolen before you could sell it," the doctor said, and he saw Kino's eyes flick involuntarily to the floor near the side post of the brush house.