Thank you for your response. I couldn't work out how to do it the way you said which did sound like a lot of manual entry based on the fact we're getting these types of deliveries on a regular basis at present. 

However I've come to a sort of compromise and found a "solution" to it where I can at least allow receipts team to manually enter the amount on creating a new "receipt" entry. This allows them to enter a quantity, and allows them to continue the process. It doesn't seem to have a way to track what was originally on the delivery note but at least I can give them a target when validating. 

The way I gained access to unlock the quantity ("done") field was as follows:

Inventory -> Overview -> click 3 dots on overview to access sub menu -> click configuration -> check "Show detailed operations"

Design an import template file to update inventory receipt operations with source location, destination location, products and the quantities. Whenever you receive unexpected stock, update the template and import and manually validate it.