Posttax discounts in invoices

[Roman Bertle]

Hello,

for invoices it is possible to set a discount for each entry. There are
three ways to treat a discount regarding taxes, depending on how
discount_how is set: PRETAX, SAMETIME and POSTTAX. A little example:
Lets consider an invoice with 10 items, the price is $100, which gives
as raw amount $1000. Further lets assume that the tax is 10%, and that
the given price is without taxes (taxincluded is 0). The discount shall
be $10, which means that the discount_type is VALUE. Depending on
discount_how, we get:

* For PRETAX, the discount of $10 is subtracted from the raw amount,
  which yields a net amount of $990. The 10% tax is $99, and we get a
  gross amount of $1089. This is the way Gnucash does it.

* For SAMETIME, the discount and the taxes are calculated independently
  from the raw amount. The net amount is $990 after subtracting the $10
  discount, but the 10% tax is taken from 1000%, which is $100 taxes.
  The gross amount is $1090, and again Gnucash does it this way.

* Now for POSTTAX, IMHO, first should be calculated the gross amount
  without discount, which is $1000, plus 10% of $1000, i.e. $1100. Now
  we subtract the discount of $10, and we get the final gross amount
  $1090. Finally to get the correct net amount, we invert the equation
  gross = (1+perc/100) * net, and get net = gross / (1+0.1), which is
  $990.91, and the tax is $99.09. But Gnucash does not work this way, it
  gives the same result as for SAMETIME (I use 2.2.3-1)!

Hence first my question: Why does Gnucash the same for SAMETIME as for
POSTTAX, is there something wrong for my considerations?

And a remark: For SAMETIME, in the end the paid tax is not 10% of the
net amount. This can be o.k., e.g. for a discount which cannot be taxed.
But for POSTTAX, I expect the tax rate to be 10%. For example, if one
gives a customer a discount for the paid gross amount, this should be
taxed correctly.

Best Regards, Roman