Hi Andreas,
With the item type : Service - fixed price (project) then any way system will consider @ the fixed price model but in case the item type is time and material (project) then you will tell the system what amount of project of time & material is involved for the project.
When the project invoice is completed then do you still see Invoice Preparation: Not Invoiced Time and Expenses ? I assume it as no
Then when the Customer Project Expense List also posted, based on your accrual method anyway you can still hold under Deferred Items by choosing the Accrual Method that supports to defer the amount and can recognize it only when the service is delivered, so that Service Confirmation will not recognize revenue straight away when you post the invoice in the beginning.
Thanks
Lokesh Sharma