Negative tax rate

Hi,
i need to set a negative tax rate, but the system ignore it!
I looked all the afternoon long for the code that calculates it ... but i couldn't find it :-(

Thanks

You need this why?

Because in Italy, if you sell services and you are a "libero professionista", you have to reduce the import of a 20% for the italian customers.
The customer will pay that amount directly to the Treasury. This is crazy, I know ...

We here at spain have the IRPF, same thing, have u solve it?

Thanks!

SOLVED!

OPEN: catalog/model/total/tax.php

SEARCH FOR:

if ($value > 0) {

CHANGE TO:

if ($value != 0) {

DONE!

That is a relief. The title suggested that someone has a government that refunds rather than collects sales taxes in advance.

There are other ways to interject an offset, but what you have seems to work, and simply, so go with it.

OpenCart 2.3.0.2. I need to show full tax rate by product and then a new line with 50% of that tax deducted. I have created tax rate with full amount and second rate with half that. Have set it so that both rates show (using store address and customer address). But can't use a negative percentage.
A message from Dec. 2013 showed changing OPEN: catalog/model/total/tax.php
if ($value > 0) {
CHANGE TO:
if ($value != 0) {
the files in 2.3 are different and I think the equivalent is system/library/cart/tax.php but not sure and not sure what I would change. Or is there a better way?

I found the file catalog/model/extension/total/tax.php and made the change previously suggested.