It would behoove you to keep it. Reason being, a unit in your care was issued before all required testing was performed. Even if the blood was returned, you need to keep the documentation as to why it was released from your electronic inventory record.
Now, say you issue a cooler full of an MTP pack and as the nurse is walking away, they cancel the MTP. The nurse returns the cooler, it doesn’t even make it to the floor, and you haven’t sent the slip for the physician’s signature yet. In that case, it could be a little redundant to make them sign a form for a nonexistent MTP, so I would just leave documentation of a variance from SOP document in the event of inspection with the documentation that an emergency release was initiated at the request of the physician and was canceled so you did not send a form.