Hi Mohamed
I don't understand exactly requirement, but maybe this will help:
Change the PAI to this:
FIELD input MODULE check_input on request.
FIELD listbox MODULE check_listbox
FIELD ...
In the check_input module you add a leave screen at the end of the module.
With this modification the check_input module is only executed when the user enter or change content of field input. The leave screen will make the process jump directly to the PBO without executing the check_listbox module.
So the program flow will be
1) user enter value in input field and press enter
2) module check_input is executed
3) PBO is executed and the screen is presented to the user
4) user select an entry in the listbox and press enter or save or what-ever
5) module check_listbox is executed
6) ...