Question how to sum a colum

Hello together,

i am a PI beginner and i need your help to a simple question:

I have tow tables (item and sales).

I want to sum up the invoice-amout out of the table sales 
on each item in the table item.

But the result is the complete sum of all items in each itemline = 1030)

The correct result must be in sum:

item 123 = 800

item 456 = 110

item 789 = 120

BR

Piet