The devil and his grandmother

The soldiers in our story were not happy with the payment they received from the king so they deserted. Suddenly a flying dragon showed up from nowhere. He was actually the Devil himself. He was not going to kill them. Instead he offered them service for seven years. In return they were going to receive a lot of money. There was however a catch. Their souls were going to remain in the Devil's hands unless they could solve his riddle. Will they succeed or it is absolutely meaningless to compete with the Devil?