$currunt_nid = $entity->nid; // Get current nid
$query = new EntityFieldQuery;
$result = $query
  ->entityCondition('entity_type', 'node')
  ->propertyCondition('status', 1)
  ->fieldCondition('field_invoice_ref', 'value', $current_nid, '=')
  ->execute();
print_r($result);