SD documents not clearing between them in IS-U invoice

Hello,

I face a problem right now that I didn't yet found a solution and maybe you can help me.

We managed to collect the SD Doc on the IS-U invoice( from billing) but the problem now is that the SD docs not clear between them.

For example, I have a SD doc with + 220

                    I have a SD credit memo -110.

I want to see only the difference between them, ==> 110.

Do you know where I can customize this or an event where to develop this functionality?

Thanks a lot!