Offset taxes functionality in Payables 11.5.10.2

Hi,

We have created offset tax codes and attached them to VAT tax codes so that in the invoice distribution when an VAT tax code is attached to the line type and click on the calculate tax button, a positive tax line  and negative tax line should be generated so that the invoice header amount is equal to the invoice distribution. 

Let me explain it with an example -

Tax code name Tax type Rate         Offset tax

Input Tax VAT 10 Offset Tax

Offset Tax Offset -10

So when we create an invoice and at the line item in the invoice distribution when we attach the 'Input Tax' and click on the calculate button, assuming the invoice amount is 1000 EUR, a positive tax line for 100 EUR and a negative tax line for -100 EUR should be generated, making the invoice distribution total equalling the invoice header total.  But its not working.  Can anyone please help am i missing any setup here.