Step 1: Situation.
Dr. Sadao, though loyal to his country, saved the life of the wounded American soldier.
Step 2: His arrangement.
He arranged a boat with essentials like food, water, and warm clothes.
Step 3: Escape plan.
He instructed the soldier to wait at a nearby uninhabited island for a passing American ship to rescue him.
Step 4: Conclusion.
Thus, Dr. Sadao ensured the soldier's safe exit without betraying his humanitarian values.
Final Answer: \[ \boxed{\text{Dr. Sadao ensured the soldier's safe escape by arranging a boat, food, clothes, and a rescue plan.}} \]