Sum of a field existing in the group header

[bilalo]

How to find the sum of a field existing in a group header and show this sum in the report header without using DSUM?

[ranman256]

the report sums as it passes the detail, so field source: =sum(field) goes in the footer.

[bilalo]

I have a relationship between two tables: Invoice and InvoiceItems where each invoice includes many sold items without specifying their prices. The NetTotal of the invoice is stored inside the invoice table. I created a report that shows the invoices in the group header and their items in the details section. In my case, I only have two invoices, my first invoice has net total 1500 and second invoice has net total 450. But the sum in the report header using sum(NetTotal) is 9300.
I don't know where the number 9300 comes from. I think it is adding the net totals multiple times according to the number of sold items in the details section.