Hi Simona
System make it as per planning in kp06 and you may know that you can plan activity dependent and activity independent planning. In activity dependent planning only you can define F&V. When you post actual exp in FI system take the planning logic to break it F&V only in case of activity dependent planning CC level. The same exp may be planned in oth CC without activity and in that case system will take it Fixed only.
I think your point is clear now. Please close the thread with proper marking as correct/helpful so that others can get help.
Kamal