If all else fails, look for the answer on google:
http://www.sqlteam.com/item.asp?ItemID=3856
aparently "ON A1.oid <= A2.oid" should have been:
"ON A1.oid >= A2.oid".
SELECT A1.oid, A1.concepte, A1.deure, A1.haver,
sum( COALESCE( A2.deure, 0 ) -
COALESCE( A2.haver, 0 )) AS value_sum
FROM Assentaments AS A1
INNER JOIN Assentaments AS A2
ON A1.oid >= A2.oid
AND A1.numero = A2.numero
WHERE A1.numero=11189
GROUP BY A1.oid, A1.concepte, A1.deure, A1.haver
ORDER BY A1.oid;