In baseball you can make a trade where the return is a "player to be named later." From what I know, this is basically one team saying "you can have this guy for now, we'll figure out what we want back at some other time." Although unlike "future considerations" in hockey (effectively legalese for "for free" or something trivial including some funny little things), they do eventually figure out what goes back in return.
Harry Chiti was once traded for a "player to be named later." However, the two teams agreed to complete the trade 15 games later by sending him back to his original team. By a creative stretch of definitions, he was technically traded for himself. I wonder how that one felt for his ego.