Report Builder 2.0 - SSRS - filtering in Row Groups: how to show only filtered values in the totals?

Question

I have an aging report with totals per patient and per month.

I need to have months as column groups and patients as row groups.

plus I need to filter out patients, whose totals overall (for all months) were negative.

I created a Row Group filter per patient with condition SUM(Totals) < 0. However, the row that shows the Grand total of all the table, still counts the values which were filtered out.

Is there any possible trick to make totals account only for filtered values?

Thank you very much!