I think you could use a formula like the following in the detail section:
whileprintingrecords;
datevar array holidays := [date(2005,01,01),date(2005,07,04)];
//add your business holidays into the array
numbervar cnt;
numbervar sumsales;
if not({table.date} in holidays) and
not(day({table.date}) in [1,7] //add your non-business weekdays
then cnt := cnt + 1;
if cnt in 1 to 15 then
sumsales := sumsales + {table.sales};
Then use a display formula like the following in the report footer:
whileprintingrecords;
numbervar sumsales;
This assumes that your report has a selection formula limiting it to one month.
-LB