Hi jacklcs
I have come across this issue (or a similar one) especially where you have a Group on the report.
My work around was to put a new attribute in the BO e.g. Account.Diff and a rule in the BO, for example:
Account.Diff=Account.DebitAmount-Account.CreditAmount
Then in the report just do a SUM Account.Diff
It works for me - good luck.