Duty is payable on 'transaction value', which has been defined as the price actually paid or payable. Obviously, discount is not part of the amout 'payable' by the customer. The price actually payable will not include the discount.
Therefore, if you are allowing 10% discount, you will have to pay duty only on the remaining 90%. This has been clarified by the TRU letter dated 30.06.2000.
" duty of excise levid on transactional value which is define under cenvat credit rule. if achual discount has been paassed by mfger to the seller then duty should be calculate on 90% of sale value