Mod Bill to take amount AND balance into account when getting not_billed

This is at the request of the AR office. I asked if this was leading us to misuse the balance field, but they were adamant that this was the usage they desired, and that not_billed was not used anywhere else so that we would be fine to make modifications.

As such, I doubled up the SQL since the SUM calculations are different, and I feel that at some point, we may roll the balance portion of the equation back out (just a hunch).