Hi Nikhil,
As you mentioned - "when the User click on Save Button" - Use this point to implement the custom logic in ORDER_SAVE BadI. In the BadI Implementation you can check the INPUT Fields which will be filled if the Text are changed and hence will let you to put your application or business logic in a much more clean manner than writing code in the presentation layout.
We should always avoid putting business logic in the presentation layer and use the enhancement framework provided by SAP.
Let me know if you need some inputs in terms of technical code reference.
Hope this helps..
Best Regards,
Samantak.