Here is a unit we have set up in Dealership Software. So far no internal invoices have been created for it.
1. Create a new customer invoice for the "Internal Customer" with the unit as the vehicle.
2. Create the invoice for the work done on the vehicle.
3. Process the invoice and press cancel when it asks for a payment (you don't need to create a payment as it's a $0 invoice).
4. The unit cost will now include the price of that invoice.