Customer is having old Purchase orders created two years ago with status picked when he tries to cancel these shipped lines of the transfer order, he is getting an error message " the quantity can't be reduced."

A bug has been raised for this issue Details for issue 795416 (dynamics.com) and confirmed it will not be fixed, our aim is to have this issue resolved in future version or by allowing a feature to determine that either. As a result, we can only set the status back to Picked.