Not able to edit Purchase Invoices in new layout.

Our purchase invoices are entered with a due date for payment for the manager to monitor ready for an end of month payment.

The trouble is our invoices are being paid to the far eat and therefore the exchange rate changes daily. The amount entered at the start can change slightly or greatly but the manager is still able to monitor the invoice we have received and keep track of orders this way.

Usually we would enter the invoice at the the exchange rate of the time the invoice arrives then the amount would change when we allocate a payment to it. This has caused issues now for ongoing monitoring for the department who deals with importing of goods.

Other than now deleting all the invoices (20+) is there any way we can change these ready for allocating the payment.