If the invoice is not created based on forward from a customer shipping advise (CSA), the quantity can be changed. If the invoice is created from a customer shipping advise (CSA), you cannot change the quantity, as the NETQTY is already reduced on the customer shipping advise (CSA). If it is necessary to change the quantity in those cases, you will have to delete the item on the invoice and forward the correct quantity from the Customer Shipping advise (CSA-Item) again.