You might be able to do this using a variable....I don't guarantee this will work, but you could try something like this:
Numbervar TransTotal;
If PreviousIsNull({table.subtotal group field}) then TransTotal := 0;
If NextIsNull({table.subtotal group field}) or ({table.subtotal group field} <> next({table.subtotal group field})) then
TransTotal := TransTotal + abs(sum({table.field to total}, {table.subtotal group field});
TransTotal
-Dell