Failing join with set returning function

I've written a function that calculates the number of days for every month in a given range and returns that as a set of records.

CREATE OR REPLACE FUNCTION general_daysinmonth(
  date1 IN date,
  date2 IN date,
  month OUT date,
  days OUT integer) RETURNS SETOF record AS
$body$
DECLARE
  startdate date;
  enddate date;
BEGIN
  IF date1 >= date2 THEN
    startdate := date2;
    enddate := date1;
  ELSE
    startdate := date1;
    enddate := date2;
  END IF;
  month := date_trunc('month', startdate);
  WHILE month <= enddate LOOP
    days := LEAST(general_lastdayofmonth(month), enddate) - GREATEST(startdate, month) + 1;
    RETURN NEXT;
    month := month + interval '1 month';
  END LOOP;
  RETURN;
END;
$body$
LANGUAGE 'plpgsql' IMMUTABLE RETURNS NULL ON NULL INPUT SECURITY INVOKER;

Now I want to use that resultset as part of a join with a table called billing (see definition below).

This join should return a record per month that is between salesstartdate and salesenddate and lineamountmst should be divided pro rata the number of days in each month

 

An example

If table billing holds a record like

id invoiceid inventtransid invoicedate dataareaid lineamountmst dimension itemid salesunit issues salesstartdate salesenddate salesstopcode salespoolid
1170 22428431 735706 2006-02-28 hlm 89,89 nlaatoal-6 nlaatoal 3m 0 2006-02-24 2006-05-23 SWI-TRM-1Y aans

Then select * from general_daysinmonth(date '2006-02-24', date '2006-05-23')