Hi Thanks for your response,
unforunately the expression you suggested gives me the exact same result as the one I was orginally using.
Although your expression looked like it would work, It still gives me the same result
What I'm trying to do is for example if I have 5000 invoices, I want to sum the margin of the negative margin invoices as a total not a negative margin of a particular line within that invoice.
Thanks for your help