An IDoc cannot really do more than a user can do online.
If you change values in FI then you create an inconsistency between FI and MM,
Anything regarding stocks has to be started in MM and along with it in the background you get the FI documents. If you change values in FI then there is nothing in the background that could change MM, hence you create an inconsistency which can be seen in MB5L transaction. Just search SCN with the keywords MB5L difference and you can learn a lot from mistakes.
In which country is this kind of business case and legal requirement?