Using Query Designer Conditions + structure

Hi everybody,

I´m, using a BEx Query concerning oustanding depts. (Query Designer: 7.x)

In the columns it shows only the customers.

In the rows i´m using a structure with key figures and formulas :

I need the following conditions:

1. Outstanding depts > 0

2. overdue: TOP 20 customers (without negative depts)

If I create both conditions, the result are 19 customers, because one of the TOP20 has negative outstanding "depts".

I need to exclude the negative "outstanding depts" from the TOP20 of overdue, so that the result are 20 customers.

Is there a possibility to achieve this goal by using conditions of the Query Designer or by creating another formula which excludes negative depts?

Thank you!