Converting stored procedure from mySQL

Hello,

I'm trying to convert one stored procedure from mysql but I have some problems to get it runing.
It's quite basic but I haven't found the answer in the docs or on the mailing lists.

So here is my function

CREATE OR REPLACE FUNCTION  sp_order_amount_per_office(accountingid IN INTEGER, code OUT VARCHAR,officetotal OUT
NUMERIC)as $$ 

BEGIN

Select office.code as code, sum(transaction.countervaluecustomer) as officetotal

        FROM office

        LEFT OUTER JOIN receipt

        ON (receipt.officeid=office.officeid)

        LEFT OUTER JOIN transaction

        ON  (transaction.receiptid = Receipt.receiptid) and (receipt.accountingid=accountingid)

        GROUP BY code order by code;

END ;
$$ LANGUAGE plpgsql;

I can run the select query in pqAdmin and it give me the correct result, but then I try to run it inside my function as
Ialso can run independently I get the following error message 

ERROR:  syntax error at or near "$1" at character 25
QUERY:  SELECT  office.code as  $1 , sum(transaction.countervaluecustomer) as  $2  FROM office LEFT OUTER JOIN receipt
ON(receipt.officeid=office.officeid) LEFT OUTER JOIN transaction ON (transaction.receiptid = Receipt.receiptid) and
(receipt.accountingid=$3 ) GROUP BY  $1  order by  $1  
CONTEXT:  SQL statement in PL/PgSQL function "sp_order_amount_per_office" near line 16

It's my first day with postgre so it is probably something very simple

Regards

Andreas