Hi @tomekk!
Don’t get discouraged! It can be pretty simple with KNIME. Like this:
In case I haven’t missed something of course 
And don’t hold it against @armingrudd and @mlauber71. They like KNIME nodes so much they can’t get enough of it 
Now you can use some rule based node to see which invoice exceeded a certain project sum 
2019_01_23_Group_Cumulative_Sum.knwf (15.8 KB)
Have a nice day,
Ivan