Xero is an Invoicing Tool which you could integrate with Freshdesk. Using this integration, you would be able to view information about the invoices sent to the requester of the ticket, within that ticket's details page.
You could track time for tickets in Freshdesk and send invoices for support, using the Xero integration.
You could integrate Xero with your Freshdesk Account, by navigating to Admin -> Su -> Apps -> Get More Apps-->Time Tracking & Billing-->Xero-->Install.
You would be asked to login to your Xero Account to authorize the integration.