Adding discount on invoice breaks invoice

I want to add a discount to my invoices in absolute numbers, not in %.

So, now I have to calculate what % a particular fixed number equals. Annoying.

I did just that and then saved my invoice, only to find two discounts on my invoice; a "Line discount" and a "Discount ()", with different amounts.
Then, going back to remove the discounts, I can only see one, and the total of the invoice is not correct anymore, seemingly using the second, now hidden, discount to calculate the total.

The only recourse I had was to delete the invoice, and record the thing all over again.

Very annoying.