What is the right thing to do?

Your answer is correct because you must give way to the green car when turning left at this intersection.

In German traffic rules, vehicles going straight have the right of way over vehicles turning left. The green car is driving straight, so you, who want to turn left, have to wait and let it pass first.

The red car on the left is waiting at a junction and does not have priority over you or the green car. Therefore, you don't have to specifically allow the red car to pass first.

Remember this rule: When turning left, always give way to oncoming traffic going straight or turning right. This helps you make the right decision in similar situations in the future.