In O. Henry's "The Gift of the Magi," is Della's character dynamic or static?

A static character is one who does not change his or her mind or opinion throughout a story. A dynamic one does. Characters usually undergo a dynamic change when they learn a lesson that contradicts what they believed in the beginning. Della is a dynamic character because she learns a valuable lesson about the price of love.
Della is a young newlywed who feels the weight of her husband's salary being cut from $30.00 a week to $20.00. Della only has $1.87 to spend on a Christmas gift for Jim, which she does not feel is not sufficient to buy a proper gift for her husband. As a result, she cuts and sells her hair to buy him a more expensive gift—a gold chain to match his gold watch. She discovers her husband sold his watch to buy her tortoise shell combs with bejeweled rims for her hair. Jim reacts by saying,

"'Dell,' said he, 'let's put our Christmas presents away and keep ’em a while. They're too nice to use just at present. I sold the watch to get the money to buy your combs.'"

The irony in this scene provides a setting for Della to learn Jim loves her more than he cares for expensive gifts. He doesn't get angry; he chuckles. By selling his watch to buy the beautiful combs, Jim proves he is willing to sacrifice material objects for Della's happiness. He values Della's happiness more than his prized possession. Therefore, Della is a dynamic character because she learns she didn't have to sell her hair to obtain money to buy her husband an expensive gift—their love for each other is enough.