Hallo we are implementing v8 with Hr_expense and analytic_accounting, and we invoice the customer on expenses.

Is there a direct path to relate an invoice line to the expense line that generated it?

UPDATE 

In reply to comments, to invoice expenses to customers I use the standard flow provided by the analytic accounting:

In the Contract form, in the line of "Has expenses" I click on "Expenses"link, I select the expenses lines to invoice, and I click on "Create invoice".