Estimated costs vs. Actual costs
An order can have estimates and actual costs.
When an order is linked to a budget, the estimate and the sum of the actual costs must have the same sign (+ or -). If you want to adjust the actual costs, you can add costs with an opposite sign as long as the Total actual costs still have the same sign as the estimate (see case 3).
If the order is not linked to a budget, these signs can differ: