Why is a requisition line unavailable for edit (locked) when line status is partially received?

A requisition line is an 'amount only' and has been dispatched to the vendor.  The total receipts for this line are about $200 less than the total amount and the status of the line is 'partially received'.  (Status of the requisition is also partially received.)  Why is the requisition line locked and not available for edit?

All receipts have been processed and related vouchers have been posted & paid.  PeopleBooks provides these two reasons a user cannot edit a requisition line but I don't believe a 'sourcing event' is in process.  We do not use Inventory or Services Procurement modules.

• Sourcing is in process for the requisition line.