Coupon deduct from Total, including VAT (Open Cart)

Our Coupon system needs a small adjustment. What we want to achieve is, that a coupon should reduce the total cart amount including VAT. We have already changed the sort order to put the coupon right before the Total. For coupons that have a smaller amount than the overall cart value this works just fine. for example (we have already changed our prices to show with vat included): sub-total 38,80€ Incl. VAT. 19% 6,19€ coupon -10,00€ (Coupon value: 10€) total: 28,80€ BUT, if the coupon value is higher than the total it looks like this: sub-total 38,80€ Incl. VAT. 19% 6,19€ coupon -35,51€ (Coupon value: 38,80€) total: 6,19€ We are using OCIE 1.2.4 which is a fork of Open Cart 1.4.9.1 and almost similar to it, there are just a few legal adjustments for german law. As this is very urgent, it needs to be done TODAY within the next few hours. Please, only bid if you are able to do it. NO TRY & ERROR. There is a lot to do on our site, so if this project is done well, we will have more jobs for you.