Discount duty type in Export Declaration

Hi Team,

We have copied existing Discount duty type  and created a new duty type and assigned to the calculation procedure.  Now the Discount duty type is getting displayed in the export declaration but amount is not getting discounted from the total value.

Example :

Value of goods          400 USD

Transportation cost   300 USD

Discount                  100 USD

Total   should be       600 USD but it is displaying  as 800 USD. 

Please suggest.

Regards,

Chalapathi G.