I would speak to your business partner, as you can amend the XML to make it sortable, however the fact that its been set to IsSortable="false" says to me that there is a reason you cant sort on it, though it seems to work for me on demo data. It might be worth asking your Business partner too to call Sage200 Support to ask why. There may be a performance issue or something like that, which is why they've disabled the sort.