Hi Saurabh,
1. If you want to store difference as a string then use any text data type.
Example : LANGUAGEINDEPENDENT_EXTENDED_Text.
2. If it is a Custom Screen you can add custom Button. If you are working on standard screen, i don't know that screen will allow you to add custom Button or not. If it is allowing configure custom action to that button.
If not supporting then write code in after modify script (remove action script) when ever data change it will give the result.
Regards,
Shiva G