Della quickly explains that she cut and sold her hair because she wanted so badly to buy him something nice for Christmas. She also assures him that her how grows very fast and that she's bought him a really lovely gift with the money she got for her hair.
This is ironic because Della has cut off all her hair and sold it to buy Jim a gift. She sacrificed her most important possession for Jim. ... Della gave up her prized possession, her long hair, to buy a fob chain for Jim's watch. Jim gave up his prized possession, his gold watch, to buy combs for Della's hair.