If an invoice in Project is to be sent in the customer's currency, this can be done by setting the Project Debtor to a different currency than the company currency. The project currency has no influence on the invoicing currency. It is simply an information field. This also means that co-invoicing can be done on a main project if desired.
All items on the project are saved in the company currency. The conversion from Debtor and Creditor currency is done in connection with Sales and Indkøb.