As Savatage said, this is not possible as you want. If you want you have to make some customization.
You need to remove two lines from OnValidate on 'Line Discount Amount' and 'Line Discount %' fields for 'Inv. Discount Amount' calculation. Also, you need to modify next Code Units (remove 'Inv. Discount Amount' calculation):
-Release Sales Document
-Sales-Post
After that you can first run Function->Calculation Invoice Discount and after that add Line Discounts.
Or you can create completely new functionality.
But I think, it is the best solution to use standard NAV functionality and explain to your customer process of discount calculation in NAV.