Excise Calculation in Sale invoice for Trader.....

Dear All,
I have created a company with Excise Type Trading in Basic Company Information.
In the same I am purchasing the material and selling the same material to the customer without any manufacturing Process. In the purchase of the material we are receiving the Cenvat credit which is stored in RG23D register. We have to pass on the same credit to the customer at the time of sale Invoice.
In the system at the time of making the sales Invoice we attaching the Item Sale Tax group. We required that the Excise duty should be pass on which we have received at the time of purchase and not as per formula design. But in the system when I am attaching the Item Sales Tax Group it calculating the Excise as per formula only and if I don’t attach the Item Sale Tax Group then it’s not calculating the Tax at all.

Can anyone guide in this regard how to pass on the same credit that received at the time of Purchase whatever may the selling Price.

Jinesh Shah