When a "Volume Discount" is set for a particular product/pricelist, the system calculated volume discount is applied even when with the Pricing Model is set to Override Price.
It is very difficult for the user to get to a desired Extended Amount without extensive trials/errors of changing the unit price.
The expected behavior would be that, when the Pricing Model is set to Override Price, the user should be able to change/set to 0 the Volume Discount.