OK. I see what you're trying to do here.
You don't need a subreport at all!
1) Insert a Crosstab and place it on the Report Footer
2) Use the username field as the 'Row'.
3) Use the Item field as the 'Summarized Fields' and set its summary operation to 'Count'.
4) Use the Amount field as the second summary and set its operation to 'Sum'.
5) Right-click one of the summary fields > Select 'Summarized Fields Labels' > Summary Horizontally. This places the second summary beside the first and not below it.
6) To show the summary labels, Right-click one of the summary fields > Select 'Summarized Fields Labels' > Check Show Summarized field labels.
7) Go to the Crosstab Expert > Customize Style tab > Turn off the Gridlines if you don't need them.
I hope this helps.
-Abhilash