Automatically set PO "Final invoice" indicator

In contrast to the delivery completed. indicator, the final invoice      
indicator is not set automatically for purchase order or invoice         
documents: it can only be set manually..."                               

Alternatively, you can use mass PO maintainence transaction tcode                        
MEMASSPO or BAPI_PO_CHANGE to set the field 'final invoice' for                          
purchase orders. For more information on these transations please                        
refer the online help.