They want to see the statement either consolidated, or excluding departments 38xx, so hard-coding the formula the way you propose won't work.
What I am going to do is to create a Department Mask "????", called "All but 38xx" (although it is really all departments), and then build this formula into a hidden cell (let's say cell A3):
=IF(TRIM(DEPARTMENT(0))="*",0,1).
The result of this formula will be 0 if the statement is consolidated, 1 if the statement is not consolidated.
Then I am going to duplicate the contents of Column B to Column C, replacing the "-`" in each formula in Column C with "-38??"
Then I can use your formula, revised like this:
= GLAMT("M",$B13) - (GLAMT("M",$C13) * $A$3)
When they select this department group, it will subtract out the 38xx figures.
We will see if this does the trick.
Steve