Let’s set aside the habits or the laws in the various countries,
what you say has sense, and I’ve done it before.
To discount a taxed price until it matches a number that sounds good in the customers ears,
that’s ok. I would not have in the ERP, but that’s ok.
What you market to your customer and what you save in your database is kind of different,
and if you really care that when you say “-10 euros”, you can make that happen with math.
It’s important that an invoice solution has right number in the DB.
Adding discounts after taxes is wrong because the tax amount in the price does not match the net amount.
So you are giving more to taxes in the price, unless you reverse the math and keep the difference, while sorting your taxes.
But the data in your database are off, then. It’s making your life more difficult.
Am I making myself clear?
I saw your PR, and it’s frankly how I would have done it. We can recover it, tweak it and test it.
It matters that we all are on the same page.