Debugging on CCGV Oscommerce

Debugging on OSCOMMERCE MS2 : CCGV contribution The french legislation is specific and I would like the coupon to appear excluding VAT (it is possible in admin, I can obtain 2 € -> 5%) but need the VAT to be re-calculated. That means that the 0,39 € (in the following example) which correspond to a VAT deduction on the coupon should be directly reduiced from the VAT amount, ie to obtain 8,82 € - 0,39 €. I tried many modifications on the [login to view URL] and [login to view URL] files but without success. What I can obtain: When Include Tax = True and re-calculate VAT on Credit Note or Standard Sub-Total Excl. VAT: 40.00€ Coupon Code:VIP: -2.39€ Shipping (Forfait): 5.00€ Total Excl. VAT: 42.61€ VAT 19,60%: 8.82€ Total Incl. VAT: 51.43€ When Include Tax = False and re-calculate VAT on Credit Note or Standard Sub-Total Excl. VAT: 40.00€ Coupon Code:VIP: -2.00€ Shipping (Forfait): 5.00€ Total Excl. VAT: 43.00€ VAT 19,60%: 8.82€ Total Incl. VAT: 51.82€ What I wanna have: Sub-Total Excl. VAT: 40.00€ Coupon Code:VIP: -2.00€ Shipping (Forfait): 5.00€ Total Excl. VAT: 43.00€ VAT 19,60%: 8.43€ Total Incl. VAT: 51.43€ Have a look on the attached files.