All the narrator's talk about how valuable Della's hair is to her, especially after her eyes begin to shine brightly and her face goes pale, seems to foreshadow that Della will sell her hair to buy a gift for Jim. The narrator says,
The James Dillingham Youngs were very proud of two things which they owned. One thing was Jim’s gold watch [....]. The other thing was Della’s hair. If a queen had lived in the rooms near theirs, Della would have washed and dried her hair where the queen could see it. Della knew her hair was more beautiful than any queen’s jewels and gifts.
This comparison of Della's hair to a queen's jewels or gifts might bring to mind her desire to purchase a nice gift for Jim, something very nice, something worthy of him. She would want to give him something worthy of a king, just as his gold watch is. The only thing she possesses that could bring her such a sum is her beautiful, cascading hair.
Further, the narrator says that "If a king had lived in the same house, with all his riches, Jim would have looked at his watch every time they met. Jim knew that no king had anything so valuable." Therefore, if we figure out what Della intends to do based on the description of the beauty and value of her hair, then we can make some guesses about what Jim intends to do with his one valuable thing as well.