If you want the l_amt column in your result set, then you have to include the l_amt column in your WHERE clause ; moreover, if you want to SUM, you have to add a GROUP BY clause also.
String queryString = "SELECT l_amt, SUM(f_amt) AS sum_fine_amt FROM fine WHERE f_status='true' AND r_id=? GROUP BY ?";
Here you have to give a column name used for groupment (ie. replace the '?' after GROUP BY with a column name).