Why did the doctor refuse to treat Coyotito in The Pearl?

The doctor in The Pearl is a very greedy man.  He was also a man filled with prejudice.  When Coyotito is stung by the scorpion, Kino is desperate to get help.  However, he does not like or trust the doctor.

“He could kill the doctor more easily than he could talk to him, for the doctor’s entire race spoke to Kino’s entire race as though they were simple animals.” (Page 9)

When the doctor’s servant announces that Kino is there with the child, the doctor confirms this belief when he says,

“Have I nothing better to do than cure insect bites for ‘little Indians’?  I am a doctor, not a veterinary.” (Page 11)

Then the doctor asks if Kino has any money.  He knows that Kino doesn’t and he laments the fact that

“I, I alone in the world am supposed to work for nothing ---- and I am tired of it.  See if he has any money.” (Page 11)

Kino tried to pay for the medical services with eight misshapen seed pearls.  They were gray and ugly and not worth anything, but they were all he had.  He gave these to the servant, who presented them to the doctor.  The doctor, knowing the pearls were valueless, refused to see Coyotito.  He sent the servant to tell Kino that he had been called out to a serious case. 

Later, when the doctor hears that Kino has found the great pearl, and the pearl is worth a great deal of money, he shows up at Kino’s home.  He knows that he can get money from Kino, and convinces him that there may be something else wrong with the child.  He actually gives the baby something to make him sick, trying to get money from Kino. But Juana has already dealt with the scorpion sting and the baby is fine.