item buydown

I seem to have found a huge problem when using buydowns from the item properties / pricing for any item you may have on a limited quantity promotion. I enter the buydown price then the quantity of the items that can be sold at the buydown price. All works fine till I near the end of my supply and have seen many extra items sold at the buydown price that should not have been. As an example, if you have 2 items left at the special price then a customer comes in and purchases 10 of the items in one sale resulting in all of them being sold at the reduced price! This can and has resulted in hundreds of dollars in loses before we found out about the problem. Anyone have a solution?