Problem with LEFT JOIN + SUM(IF())

[SonyBoy34]

I'm trying to get a monthly total of invoices and turnover (from table 'invoices') and the total payments for those invoices (from table 'payments').

This is the query that I use:

SELECT 
  COUNT(i.inv_nr) as nbr_inv, 
  SUM(IF(i.cancelled='0',i.total_amount,0)) as turnover, 
  DATE_FORMAT(i.date_sign,'%m %Y') AS month, 
  SUM(IF(i.cancelled='1',1,0)), 
  SUM(p.amount) 
FROM invoices as i 
LEFT OUTER JOIN payments as p 
  ON i.inv_nr=p.inv_nr 
WHERE i.type != '7' 
  AND month = '$g_b_month' 
  AND i.inv_nr IN (
    SELECT inv_nr from invoice_details) 
  AND i.unit_ID='$util_ID' 
GROUP BY month
 

The problem is that some payments are total, or partial, or missing. When the payments are multiple for the same invoice, it multiplies the counted number of invoice('nbr_inv') and the turnover.

Does anyone have an idea?