In the case of making a sales order and making a sales invoice for a part of this order. Why is this invoice value not deducted from the order
Why should it be? A sales order is not a financial transaction. It only records a customer’s desire to buy something from you. And it serves as a convenient starting place in a workflow to enter information that will later be copied to transactions, such as sales invoices, delivery notes, or credit notes.
Nevertheless, Manager’s status-monitoring capabilities will show such a sales order as being partially invoiced. See the Guide: Monitor sales order status | Manager.