You need to fill out the blanks in your description - in particular: what is the table that you are summarizing, what is its relationship with invoices and payments, etc.
My RG in this part of the solution looks like this:
Client >> Finances >> Line Items << Product Codes
This layout summaries line items. The field is Amount Paid which is a calculation that draws in the amount from the Finances Table. It works fine. Then the summary field Amount Sum sums the Amount Paid of the Line Items table.
If I understand your description correctly, EACH record in LineItems gets the AmountPaid value from the parent record in Finances. So if the original amount was say $75, and there are 3 line items, your summary field will show 3 x 75 = $225.