If you set the server output in ON mode before the entire code, it works, otherwise put_line() will not work. Try it!
The code is,
set serveroutput on;
CREATE OR REPLACE PROCEDURE PROC1(invoicenr IN NUMBER, amnt OUT NUMBER)
AS BEGIN
SELECT AMOUNT INTO amnt FROM INVOICE WHERE INVOICE_NR = invoicenr;
END;
And then call the function as it is:
DECLARE
amount NUMBER;
BEGIN
PROC1(1000001, amount);
dbms_output.put_line(amount);
END;