Well, Melkor is the obvious choice for leaving them (the Ainur) ... although Ulmo leaves them (the Ainur) in a sense, just not in terms of abandonment. But a deal?
You're not expecting us to imagine Melkor saying 'No deal' to Eru in a cringey Noel Edmonds voice, are you?
Either that or a reference to Feanor refusing to eat his sprouts. ('He left them.')