After the loving couple have sacrificed their most prize possessions out of love for the other, proving that they understand the true meaning of Christmas, and proving,Jim tells Della that their presents should be saved until later as "They're too nice to use just at present. I sold the watch to get the money to buy your combs." Of course, Della's beautiful combs cannot be use now because she has sold her luxurious, long hair in order to procure the money with which she purchases the handsome watch chain. However, her hair will grow back to its length with the passage of time so, Jim puts them away until they can be given to his wife for her use with longer hair than she has now.
In the end, you can view by yourself as "rich" not for their wealth or possessions, but for their spirit of generosity and love.