This functionality is new as of version 4.0.
To Cancel Ordered Quantities:
Navigate to the 'Purchase Order Details' screen → 'Purchase Order' tab:
In the 'Cancelled Qty' field, enter the quantity to be cancelled.
Click the ‘Save’ button.
The 'Status' of the Purchase Order may be updated.
The rows highlighted in green are Purchase Order Items that have been ‘Received’.
Purchase Order Status Update:
'Cancelled': If all of the Purchase Order Items were ‘Cancelled’, the Status will be updated to 'Cancelled'.
‘Incomplete’: If some of the Purchase Order Items were ‘Received’, and some, but not all, of the remaining items were ‘Cancelled’, the Status will be updated to 'Incomplete'.
'Received': If some of the Purchase Order Items were ‘Received’, and the remaining items were ‘Cancelled’, the Status will be updated to 'Received'.
Also see About Purchase Order Status.