Taxes as a negative value

I've noticed that in Zoho Invoice taxes can only be positive (i.e. the tax value is added to the total, for example a 1000$ object with a 20% tax would give a total of 1200$)

In my country (Italy) we often display on invoices also negative taxes, which are paid by the seller.

So, a 1000$ consultancy could have a negative 20% tax. The total would be 800$.

Usually a 1000$ consultancy made by a professional has a +20% VAT on its price and a -20% "ritenuta d'acconto" (a pre-tax paid by the seller) on its price.
The total would amount to 1000$, but Italian law requires us to display this +20%, -20% simple calculation on every invoice.

And we cannot use discounts to obtain this, because negative taxes are required by law to be applied to the total and not to single objects of the invoice.

Summarizing: please let us use negative values for the tax % field (I've tried typing "-20" in the tax settings, but the system does not accept it).

Thanks in advance