Thank you for posting!  I was going crazy trying to close an "open" sales order.  Despite being fully invoiced the correct way - one sales order would not "CLOSE"  The amount due on the open sales order was zero (since it was fully invoiced) but the manual button to close the sales order was grayed out.  No idea what caused this, but I simple emailed the sales order to myself as suggested.  And VOILA!  problem solved.