Have you meet a situation that you delivered or received partial quantity of an order or sales quotation? For example, you sell an order which the quantity is 15, but can only send 5 items at first and the customer decide to reduce the order to 10. All the actions can be made in the original file without build a new one in SAP Business One.
As you can see, since not all the items have been delivered, the status is still open and you can change the quantity anytime. Also the open quantity will be updated accordingly.