I'm having a look in a EPi Commerce project Version R3.
I'm not sure if the previous developer change some think, or if this is the normal behaviour.
The case is. Whem I edit the meta field in the order manager, doesn't matter the field. The amount for the Balance Due change.
Some times for a positive value, some times for a negative.
Where I should be have a look about it?
Saving the order will cause some workflows to be run - in this case I suspect it to be RecalculatePurchaseOrderWorkflow.
Balance Due is calculated by AuthorizedPaymentTotal - CapturedPaymentTotal, so check if your workflow will change one of those values.
Do you know why in a finished order EPi trigger a workflow?
There are a way to configure, or change the settins for not trigger the workflow?