Invoices can be sent to Xero manually with a click of a button.
Whether your Xero settings are set to manual or automatic, as long as you do not have the same invoice pre-existing in Xero to be reconciled, then you will be able to manually send it.
Go to the Invoice and click on [Send to Xero].
ResBook will return a message saying that the invoice has been sent successfully.
If you came to a halt as you are sending, and an error message displayed onscreen, find out more on about this occurred in Xero errors and invoice rejection.