First of all, welcome to BYC! I'd recommend making a thread about this in the Emergencies/Diseases section. Maybe one of you could move it @TwoCrows@DuckLady (thanks!)
I agree that is could be frostbite which is evident by the pink skin above the sight of the frostbite. This results in self amputation in a month or so. In the picture below, you will notice the pink live tissue above the black necrotic tissue. If you have Betadine, you could spray some on the stump daily until all of the bone and tissue are gone or healed.