What connection does Momma have to Dentist Lincoin? How does he treat her and her request?

Maya has two very bad cavities, from eating too many sweets. There aren't any black dentists in Stamps, so Momma decides to take her to a white dentist in town, since he owes Momma a favor. Maya is in a lot of pain, so Momma has to drag her to the dentist right away. They get to Dr. Lincoln's, and wait on the back porch for an hour. Momma asks for Maya to be treated, but the dentist insists that he doesn't treat negro patients. Momma insists, since she had lended him money to keep his place during the Depression; he says since the money is all paid back, he doesn't owe her anything anymore.

Momma tells Maya to wait outside, while she goes in to talk to Dr. Lincoln; Maya describes a fantasy scene, in which Momma gets revenge against Dr. Lincoln and makes him apologize for his insults to her. Momma does nothing of the sort, and resolves to take Maya to the black doctor many miles away in Texarkana. Maya goes and has her two teeth removed, and comes back to Stamps and tells her brother about the "encounter" between Momma and Dr. Lincoln. As it turns out, Momma was just able to extract from him interest on the loan she gave him, and wasn't able to rebuke him for his rudeness after all.