TAX calculation problem

[ifs-net]

Hello,

I am having a tax calculation or display error on invoice...

When I change the price of an existing product it is not recalculating the tax or it is calculating as 0..

For example;
- the item price is 100 usd and the tax (vat) is 10% and in total you see 110 usd at bottom of the invoice. Means;
product price 100 + tax 10 = sum 110

OK everything looks fine when you first create a product but when you change/edit the product price (for example if you make a discount of 50%, and you decrease the price from 100 usd to 50 usd) this is what you face

- the item price 50 usd (discounted from 100 usd) with the same 10% tax in total the price is 55. BUT WITH A SMALL DIFFERENCE, you are facing the pricing below on the invoice
product price is 55 + tax 0 = sum 55


How can I fix that pls ?

*I am only facing that problem when there is child products