Using Quik Reports to negate Credit notes:

Order Entry stores the document type with the transaction. Because it knows when it is dealing with a credit note, it stores the values as positive. To get an amount or quantity field to print as a negative, you will need to enter a formula based on the document type for each of them.

This is a sample of the kind of formula you will need to use:

If Document type = 'Credit note' then - Extended invoice price else Extended invoice price