Budget amounts in acquisitions are always calculated, meaning we pull the orders and do the math. Recently we have added a few changes to how things are calculated, additionally one can edit orders after a budget is closed ( bug 10921 ) We have three unused columns: budget_amount budget_encumb budget_spend It might be worthwhile to use these (or add new) and store the values of a budget at closing to provide a constant historical value for audits.
It sounds like the problem is the option to change the values when it should not be possible. Maybe it would be more effective to not allow editing of orders that belong to a closed budget. Or only allow it when the fund is changed to an active one the same time.