Your idea is good in theory, but you risk to erode your own pricing if you do that. What would probably happen is that the advertisers wouldn't purchase the slots until the end of the period, when they know that you will decrease prices for the vacant slots. So, you risk actually getting less revenues if you introduce those discounts – and not more.
However, if you share your idea during the real case ("increase fill rate through discounts for vacant slots") but also share your concerns / limitations ("we need to be careful not to erode our pricing") – that would be an excellent point, which you should definitely make!
Hope this helps! If you want to discuss in more detail, just DM me :)