Something that you should note, is that custom field values from summary saved searches are never stored values, they are dynamic. Since they aren't stored, you won't be able to pull that custom field in a saved search. If you still wanted to have that custom field made though for item fulfillments, for the search you created to source the custom field from, have it be a transaction search, and try having Type = Item Fulfillment, and Applied to Transaction fields... Type = Sales Order. Then in the results use CASE WHEN {quantity} = {appliedtotransaction.quantity} THEN 'COMPLETE' ELSE 'PARTIAL' END.