Can you provide a more detailed example of what you are trying to accomplish?
This sounds like something where you can subtract the sum (that computes a sub total) from a grand total value, or you might simply sum the value of a sign where your field has been multiplied by negative 1 to invert its sign...
But sum can produce very different results depending on exactly what you are putting between the parenthesis.
Sum ( field ), if used exactly like that, only returns the value of Field as you are computing the sum of a single value in a single field.
Presumably, you meant: Sum ( RelatedTable::Field ) --which sums the value of Field over all the related records.
But you could also have meant:
Sum ( RepeatingField ) --which sums the repetitions of the repeating field.
Sum ( Field1 ; field2 ; field3 )