Hi Vinay,
Hope 'status' and 'outline agreement' belongs to same screen. its not possible to call the PBO after PAI in same screen, disabling screen can be done via PBO.
Instead of disabling the screen, validate with message using user exit/Enhancements.
if status = 'AALK' and outlineagreement IS NOT INITIAL.
Message 'field should be blank' type 'E'.
else status NE 'AALK' and outlineagreement IS INITIAL.
Message 'fill the outline agreement' type 'E'.
endif.
Hope it helpful,
Regards,
Venkat.V