On original Project budget when you have lines in 'Details', the field 'Original budget' is greyed out because the lines in details populates the total showing in field 'Original budget'.
However in Budget revision you are able to edit field 'Revision amount' when you have lines in 'Details'.
There should be a parameter to control if the user is allowed to edit 'Revision amount' if there are detailed entries, in order to ensure that detailes are used. Now the revision amount can be different from details and therefore can be out of sync.
This issue was reported as a hotfix request, but did not meet the bar for a hotfix.