Show in cart coupon discount not including the value of the tax

[ironclark]

Hello!

I would like to show in the shopping cart the discount for a coupon not including the value of the tax, such

Product price: $10000 tax: 16%

MY CART NOW          TAX          TOTAL
PRODUCT1               1600        11600
COUPON 35% OFF    560         4060
TOTAL                       1040        7540

CORRECT FOR ME

                                  TAX     TOTAL
PRODUCT1                 1600   11600
COUPON 35% OFF                  3500
TOTAL                        1040   7540

I´m using

Joomla 2.5.9
Virtuemart 2.0.20b

Please help me!