Many friends, while using Odoo, feel that they haven't modified anything and it's a brand new Odoo database. When testing business processes, they always encounter some incomprehensible issues. Starting from Odoo 12, we also encountered the above problems and found where the issue occurs.

The general reason for this problem is selecting the wrong accounting subject, or the accounting subject being the wrong type, which can lead to the above issue. It is necessary to go to Invoicing -> Settings -> Tax Rates -> Edit and modify to the correct subject, and then the negative number problem will not occur.