PostgreSQL using variables in FOR Loop - postgresql

Question

I have two tables:
CREATE TABLE arapply
(
arapply_id serial NOT NULL,
arapply_postdate date,
arapply_source_docnumber text,
arapply_target_docnumber text,
arapply_target_paid numeric
);
CREATE TABLE aropenbal
(
ar_id integer,
doc_number text,
doc_type text,
doc_date date,
base_amount numeric,
paid_amount numeric,
open_balance numeric
);
For each entry in aropenbal, I want to SUM arapply.arapply_target_paid values where arapply.arapply_source_docnumber = aropenbal.doc_number (if aropenbal.doctype is C or R) or arapply.arapply_target_docnumber = aropenbal.doc_number (if aropenbal.doctype is not C or R) AND also arapply_postdate <= aropenbal.doc_date. The result should be stored to aropenbal.paid_amount.
I then wish to update aropenbal.open_balance with aropenbal.base_amount + aropenbal.paid_amount.
The function should return the total (SUM) of aropenbal.open_balance.
I'm having problems with the code below. The SELECT statements inside the FOR loop don't work, unless I manually assign a value say '362' in place of r.docnumber. Otherwise, the result is zero.
Seems to be a formatting problem.
Any insights?
CREATE OR REPLACE FUNCTION testit() RETURNS NUMERIC AS
$BODY$
DECLARE
r RECORD;
BEGIN
FOR r IN
SELECT * FROM aropenbal ORDER BY doc_date
LOOP
UPDATE aropenbal SET paid_amount = (
CASE WHEN (doc_type IN ('C', 'R')) THEN
(SELECT COALESCE (SUM (arapply_target_paid)* -1, 0)
FROM arapply
WHERE arapply_source_docnumber = r.doc_number
AND arapply_postdate <= r.doc_date)
ELSE
(SELECT COALESCE(SUM (arapply_target_paid),0)
FROM arapply
WHERE arapply_target_docnumber = r.doc_number
AND arapply_postdate <= r.doc_date)
END) WHERE ar_id = r.ar_id;
UPDATE aropenbal SET open_balance = (base_amount - paid_amount)
WHERE ar_id = r.ar_id;
END LOOP;
RETURN (SELECT SUM(open_balance) FROM aropenbal);
END;
$BODY$
LANGUAGE plpgsql;