why not simply this?
$db = $this->Zaduzenja->getDataSource();
$iznosOd = $db->value($iznosOd, 'double'); // quotes and escapes input to avoid SQL injections
$iznosDo = $db->value($iznosDo, 'double'); // ditto
$zaduzenja = $this->Zaduzenja->find(
'all',
array(
'conditions' => array(
'Zaduzenja.placeno' => 0
),
'fields' => array(
'Zaduzenja.obveznici_id',
'SUM(Zaduzenja.zaduzenje) as dug'
),
'group' => "Zaduzenja.obveznici_id HAVING dug BETWEEN $iznosOd AND $iznosDo"
)
);