I do not believe that what you want to do is possible. Your best bet for help is to contact Tech Support for possible other approaches (like a DATA _NULL_ report, or making 2 passes through the data.
If PROC REPORT is doing the BREAK on the summary, you cannot suppress the break conditionally. If you are writing out a custom break line, you cannot execute the LINE statment conditionally -- you can only set a text string whose value is set conditionally:
compute after brkvar;
if sales.sum = 0 then text_str = ' ';
else text_str = 'Sales: '||put(sales.sum,dollar14.);
line text_str $char50.;
in which case SOMETHING is still being written at the break -- but not being completely suppressed.