I have a question, rather than a comment

I have a question, rather than a comment. In a retail environement, merchandise can be sold at promotional prices over given periods of time. The solution provides Inventory tracking over time. Problem is, since the merchandise can be on-promo, as soon as I add the filter to to evaluate only promo, my inventory calc does not work. Promo adds a filter that eliminates the inventory amounts for when the merchandies items were NOT on promo. How do I handle this? It's as if I have to solve the inventory amount first, then 'label' rather than 'filter' the calcutated inventory value. Though this 'labeling' changes over time as you look at history. Thoughts?