In contrast to the suggestions in the other thread, I would go for additional code that checks for increase in order quantities for delivered items and for adding items and protect that with a z* authorization object, added to roles for system users, firefighter and 1-2 persons per sales operation. The rest of the business users would just get an error message preventing them from saving the document.
For some reason I don't like the idea to lose another option for correcting problematic documents in PRD. When you give the responsibility to just 1-2 individuals in the organization, the situation is quite manageable.