What is the ruling of Islamic law on saying, "May God not grant you profit," to a person who refused to move his broken-down car, and how can one atone for this mistake after forgiveness has been refused?

It is not permissible to supplicate against a Muslim unjustly, and the sin is graver if the person supplicated against is elderly and excusable. Your remorse is a repentance between you and Allah, but the man's right is not waived except by his forgiveness. You must appease him and ask for his forgiveness. If he refuses, then seek forgiveness for him and increase in good deeds. The man did well in his response and erred by not forgiving you, and you erred then did well by returning to what is right and appeasing him.