Jim obviously realises that Stella will not have much use for his gift right now since she has sold her luxurious locks to have money to buy him a gift. Once her hair has regrown, she would be able to use this precious token of his love and love him all the more for it.
After the loving couple have sacrificed their most prized possessions out of love for the other, proving that they understand the true meaning of Christmas, and proving, as O. Henry narrates, that they "[O]f all who give and receive gifts....are the wisest," Jim tells Stella 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, Stella's beautiful combs cannot be worn presently; for, she has sold her luxurious, long hair in order to procure the money with which she purchases the handsome watch fob. 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.