optional materials

Hi All,

Is it possible to give the discount more than the price. The scenario is like this. My customer sells a material(apartment). He is maintaining this unit as a single material. A sale order is created for this. If the customer wants to change the tiles of the apartment with another material, my client bills separately for the second material without changing anything in the original order. so when i create a separate order for the second material, i'll give the price of the first material as discount. if the price of the second material is less than the first material, then i'll have to give the discount more than the order value which results in negative value for the order. how can we achive this without using varient configuration.

Regards

samsan