Client has get/setProperty() methods, but no removeProperty(). This implicitly assumes that a setProperty(key, null) is actually allowed on the underlying HashMap (which was a horrible mistake, as Josh Bloch has recently admitted).
An alternative approach could be to implement setProperty() to remove the key/value entry on null value instead of putting null into the map, but I do not know how this might might affect other xfire use cases, therefore removeProperty() would probably be the solution with the least surprises.