Weird - I found a wild baby fowlers toad with this exact problem. Seriously, the leg was held out EXACTLY like this and was unable to move (the leg) other than twitch it. It did ok, was able to swim and hop and burrow, however awkwardly. I dont think it can be cured, but it is probrably not lethal. My toad may have had a disease/parasite causing it or maybe it just got injured. I released it though after a few weeks. I would keep it and try to heal it (or deal with it) Good luck.