Hi,
For what I remember, it would simply generate a new EMF and not de-commit the old one. In order to allow negative amounts in EMF, which are created as a result from reversals, you should change the settings of EMF type in customization allowing negative amounts.
IMG - Public Sector Management - Funds Management Government - Funds Management-Specific Postings - Earmarked Funds and Funds Transfers - Define Document Types
Regards,
Eli