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.
The only foreshadowing I see in the story, "The Gift of the Magi", is when Della has been crying because she only has $1.87 cents to buy a gift for Jim. She then stands by the window, "looking dully at a gray cat walking a gray fence in a gray backyard". This foreshadows sadness to me since Christmas was the next day and she didn't have the money to buy Jim's present. Then she whirls around from the window and "her eyes were shining brilliantly, but her face had lost its color within twenty seconds. She rapidly pulled down her hair and let it fall to its full length". When she lost the color in her face it foreshadowed that she had thought of a plan but it meant having to make a hard decision.
Since it is a short story these are the only two instances of foreshadowing that I could find.