A negotiable receipt may be negotiated: (a) By the owner, thereof; or (b) by any person to whom the possession or custody of the receipt has been entrusted by the owner, or holder, if, by the terms of the receipt, the warehouseman undertakes to deliver the grain to the order of the person to whom the possession or custody of the receipt has been entrusted, or if at the time of such entrusting the receipt is in such form that it may be negotiated by delivery.