There are several ways to reduce/cancel quantity on a supplier/customer order item.

The user can forward the total item to a new document and reduce the quantity. 

We have seen examples where users forward the total quantity to a new document, cancel the quantity and then create a new item with the correct quantity or cancel the document and create a new document with the correct quantity. We have seen examples where the user forwards the quantity to a goods reception document (SSA) and changes the quantity before registering the goods to stock.

The above options will not give a good revision of what has happened.

If the user would like to keep the link to the source order (SPO or COA), and reflect what has been reduced and why, it is best to forward the required quantity to a new document item and afterwards, forward the quantity to be reduced/cancelled to the next item on the same document and correct that quantity to zero 0. 

If only part of the goods have been received at the warehouse and the rest is to be cancelled, only the quantity received should be registered on the goods reception item (SSA-item) and the remaining quantity should be cancelled on a supplier order document (SOA) if that has been agreed. 

The user can add a note explaining the reason for the reduction.