Free freight as a Promotion in addition to other discounts

We need to give multiple types of discounts in a promotion which is manually entered and includes % discount on list price, change in Payment terms and free freight. When I add 2 types of terms substitution (1 for payterm and 1 for freight) in a modifier, both the term substitution do not work. I am not sure if it is a bug or it is just an invalid case.

The other option I was thinking of was to give the free freight as a discount using the formula. However, I am not sure on how to get the exact $ amount of freight (which is dynamically calculated using other modifiers).