This is not automated. Easiest would be to create a custom field “Billable” on the expense claim and a “Customer” field for the customer to bill. And then manually create an invoice.
Solution with the task, works to the point of registering the expense in the project but then how can we invoice it?
Solution with customization would be nice (it reminds of zoho invoice) but then how functional would that be apart from just having a clickable selection that does not much anything in the schema?
The key functionality looking for here is to somehow turn an expense into a customer invoice.