I guess the issue here is that you are updating INV_DISCOUNT and the INV_TOTAL uses the INV_DISCOUNT. so that is the issue here. You can use returning clause of update statement to use the new INV_DISCOUNT and use it to update INV_TOTAL.
this is a generic example let me know if this explains the point i mentioned
CREATE OR REPLACE PROCEDURE SingleRowUpdateReturn
IS
empName VARCHAR2(50);
empSalary NUMBER(7,2);
BEGIN
UPDATE emp
SET sal = sal + 1000
WHERE empno = 7499
RETURNING ename, sal
INTO empName, empSalary;
DBMS_OUTPUT.put_line('Name of Employee: ' || empName);
DBMS_OUTPUT.put_line('New Salary: ' || empSalary);
END;