I have a relatively complex report (2 different grouping levels) and
I'm trying to get a specific sum and I'm having some problems.

I have 3 columns that are involved in the sum. PW, ceded_PW and
isInternal

The sum I want is sum(PW) - sum(ceded_pw * -1 * (isInternal - 1))
I have also tried this like this:
sum(PW - (ceded_pw * -1 * (isInternal - 1)))

isInternal is a flag that is either 0 or 1 I'm subtracting 1 so that
the multiplication will zero out any internal numbers, and only
subtract the external numbers from the table.

In my experimentation I added the PW - (ceded_pw * -1 * (isInternal -
1)) as it's own column to the stored procedure and then summed that
field and the result was correct, so I believe that scoping is not what
is causing the problem.

Any suggestions would be appreciated.

Can you write the error message you recieve?
Make sure that the isInternal field is an integer like type. If isInternal