OK, I created a calculated column called 'True amount' based on the formula shown in the above message. This now shows the credit note amounts as negative (they actually show as "(3,289.44)".
However, when I looked at the commissions to be paid in this report, even the supposedly negative amounts resulted in positive commissions being calculated. For instance, 10% commission on (a credit note amount of) "(1,000.00)" resulted in a commission amount of "10.00". Obviously, GridView doesn't view amounts in parantheses as negative amounts.
In order to get around that, I created another calculated column called '$$$' (to distinguish it from my existing '$ Comm' column and entered the following formula:

IF({Doc Type}="C", -{True Amount}*{% Comm}/100, {True Amount}*{% Comm}/100)

However, the commissions calculated in the '$$$' are still all positive. Is there something I'm missing? How do I get GridView to view amounts in parentheses as negative amounts?

thx,

A.