[accounting] New API migration

- Task: https://www.odoo.com/web/#id=9955&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.Uy4wEA7I4G4GwPaU
in fact, sign is used only to find the absolute value of amount. https://docs.python.org/2/library/functions.html?highlight=abs#abs