When you create more than one invoice for a percentage of an Estimate that includes a discount, the discount amount calculates incorrectly on all invoices except the first one.
The discount only calculates incorrectly if it is a fixed amount.
You can use a discount item that calculates as a percentage to prevent this issue in the future:
Important: When creating a new Estimate that uses a discount, select this item in place of your old discount item.
Alternate Solution: You may continue to use the Discount Item as a percentage by changing the way you create your last Invoice for an Estimate.