Order PreClosure

Hi,

Consider the following Scenario -
Say a Customer Orders for 20 units of item number 1000.
A sales order is generated for 20 units.
Later 10 units are shipped and invoiced to the customer.
After this the customer says he needs only 10 units instead of 20.
Now the sales order has to be closed as the requested quantity is 10 and already 10 units have been shipped and invoiced.
How will a situation like this be handled in Navision.
Even if the sales order quantity is changed from 20 to 10 after reopening the sales order (Functions>>Reopen) the sales order remains as it is. But actually it has to disappear from the sales order window as the order has been fulfiled.

Does any one know how this is handled.

Kindly help soon.

Regards

Raj