Want to Chang taxable amount

Hi Experts,

I have below requirement on sales order.

Requirement - Sales order amount 3000 and discount is 6 %. Once we save sales order, line amount becomes 2820(3000-180=2820) and tax will be calculated on line amount 2820 as per STANDARD-TB

But requirement is to calculate on 3000. We cannot use assessable value as Sales order import from other system and prior tax also cannot be used.

Please suggest how we can calculate on original amount?

Thanks

Swamy