Discount With Promotional Modifier

Hi,
If we are using Promotional Type Modifier , can we add discount for the item too ?

Scenario :
I am Giving buy one get one free scheme and extra 10% discount on the item .

i.e. If the item costs 100$ then after discount customer has to only pay 90$.


I used modifier for the free Promotional item ,  At the sale order level when i try to add discount by clicking action and view adjustments and add discount system is not letting me too add discount .

Can any one help me in this regard.

Thanks
Mohsin