Kino tried to pay with tiny, ugly seed pearls.
When Kino’s little baby Coyotito is stung by a scorpion, he heads to the doctor. He is a poor pearl diver, and he does not have money. The doctor complains that no one ever has any money, and he is expected to work for nothing.
When the doctor’s servant asks him for money, he brings out all he has—and it is not worth enough.
He brought out a paper folded many times. Crease by crease he unfolded it, until at last there came to view eight small misshapen seed pearls, as ugly and gray as little ulcers, flattened and almost valueless. (ch 1)
The servant returns to say that the doctor has gone out, called away to a serious case. Everyone knows he is refusing to see Kino because the little pearl bits are not worth anything. Kino is ashamed, because there is nothing he can do to save his son. Feeling angry and helpless, he punches the gate on his way out.
The doctor does not see Kino as a human being. He does not feel bad for him, or for his baby. To him, his services are just services. He should get paid. It never occurs to him to try to save the baby because it's the right thing to do.
The main character, Kino dived for some pearls and tried to pay the treatment for Coyotito. After Coyotito was stung by a scorpion.