Unfullfilling items

If an item has been fulfilled on a sales order for a particular customer, how do you "unfulfilled it?

Case 1: customer 1 orders serial numbered item, item is received, item set to be delivered 10 days from now. Meanwhile, customer 2 orders same item, so item is given to customer 2 instead. Serial number of customer 1 is given to customer 2. I need to figure out how to undo this so the right items are in the right place.

Case 2: somehow a serial numbered item was received into inventory, but no serial number was entered in the system. I adjusted the inventory in (item with serial number) and out (item without serial number). But sales order has been fulfilled with item containing no serial number. I need to re-fullfill this sales order with the item containing the serial number. When I try to remove the bad item, it tells me: