Is there a "NVA" (not valid after) on the ticket? Is there a maximum stay? Is the ticket only valid as a roundtrip (not sellable for the same price as two one-ways?) If you answered yes to any of these, there is likely no residual value, even if there would be something remaining after any change penalties, as the ticket was sold with certain restrictions which were not followed, and had specific application for which your new flight most likely will not fit into.

It's worth asking though, if you don't know the answers to the questions I asked.