I hope you're just trying to sum the Euros (that are displayed) for each Invoice Numbers (irrespective of where they come from) and show it on the Group Footer of each Product?
Since there is no Subreport 1 record in Product B's group and since the reset formula should initialize the shared variable to zero in the Group Header, I see no way the values from Subreport 1 (of Product A) should be added up in Product B.
Could you paste all the formulae - the one that adds up the shared variables, the one that displays the sum on the Group Footer and the one that resets?
-Abhilash