Using the MIN function in the having clause

How about using ROWNUM?

SELECT *
FROM(SELECT first_name, salary
     FROM Employees
     ORDER BY salary
) WHERE ROWNUM = 1

SELECT first_name, salary  as "sal" 
FROM   employees
WHERE  salary =(SELECT MIN(salary) 
                FROM   employees);

Try this solution, inspired from here:

SELECT e1.first_name, e1.salary AS "sal"
FROM Employees e1
LEFT OUTER JOIN Employees e2
ON (e1.id <> e2.id AND e1.salary > e2.salary)
WHERE e2.id IS NULL;