The substances contained in a toads skin, even if it is dead, could be deadly for your dog if he bites it. The substances have an irritant effect in the oral cavity and the stomach, provoking excess salivation, vomiting, and diarrhoea. Sometimes even shaking and convulsions could appear. It could also be harmful for the heartbeats, the dog will fell them stronger and especially for smaller dog , this could lead to death.
It is important at this point to contact your veterinarian. Meanwhile you can relieve him by putting compresses that have been soaked in sodium bicarbonate diluted with water, on the tongue and gums. By doing so you will clean the dogs mouth and eliminate the maximum of the toxic substances.