step 2

          <sales_order_invoice_save_before>
            <observers>
              <training_orderIntegrationClient>
                <type>singleton</type>
                <class>training_orderIntegrationClient/observer</class>
                <method>salesOrderInvoiceSaveBefore</method>
              </training_orderIntegrationClient>
            </observers>
          </sales_order_invoice_save_before>
          <sales_order_invoice_save_commit_after>
            <observers>
              <training_orderIntegrationClient>
                <type>singleton</type>
                <class>training_orderIntegrationClient/observer</class>
                <method>salesOrderInvoiceSaveCommitAfter</method>
              </training_orderIntegrationClient>
            </observers>
          </sales_order_invoice_save_commit_after>

---------------------------- Observer.php -------------------------------------------------------

    public function salesOrderInvoiceSaveBefore(Varien_Event_Observer $args)
    {
        /** @var Mage_Sales_Model_Order_Invoice $invoice */
        $invoice = $args->getInvoice();
        if ($invoice->isObjectNew()) {
            $order = $invoice->getOrder();
            if (! $this->_getClient()->canInvoice($order)) {
                 Mage::throwException("Synchronization Error: invoice capability not available for order {$order->getId()}");
            }
        }
    }

    public function salesOrderInvoiceSaveCommitAfter(Varien_Event_Observer $args)
    {
        /** @var Mage_Sales_Model_Order_Invoice $invoice */
        $invoice = $args->getInvoice();
        $order = $invoice->getOrder();
        if ($invoice->isObjectNew()) {
            if (! $this->_getClient()->invoice($order)) {
                Mage::throwException("Synchronization Error: unable to synchronize invoice for order {$order->getId()}");
            }
        } else {
            $this->_getClient()->push($order);
        }
    }

------------------------------------------------ Client.php -----------------------------------------------

    public function canInvoice(Mage_Sales_Model_Order $order)
    {
        if (! $order->getId()) {
            Mage::throwException("Unable to check capability for unsaved order model\n");
        }
        return '1' === $this->_send("availability {$order->getId()} invoice");
    }

    public function invoice(Mage_Sales_Model_Order $order)
    {
        if (! $order->getId()) {
            Mage::throwException("Unable to invoice unsaved order model\n");
        }
        $this->_send("sync off");
        //$result = '1' === $this->_send("invoice {$order->getId()}");
        $result = '1' === $this->_send("pull {$order->getId()}");
        $this->_send("sync on");
        return $result;
    }