It appears that you would need to make a 'running total' from the value passed back from the subreport. I have tested the following code, so it may not work.
In the header of the report put a formula something like this.
shared numbervar Net_change_total :=0;
In a section following the subreport put a formula something like this.
shared numbervar Net_change;
shared numbervar Net_change_total;
Net_change_total := Net_change_total + Net_change;
In a section where you want the total to appear.
shared numbervar Net_change_total;
Net_change_total;
Of course the important formula in the subreport.
shared numbervar Net_change := (running total value);
I hope this works.