What is the right thing to do?

You have correctly chosen to give way to the green car.

According to German traffic rules, when you want to turn left, you must yield to oncoming traffic that is going straight or turning right. In this situation, the green car is coming from the opposite direction and is driving straight, so you must give way to it before you turn left.

The red car on the left side is parked or stationary, so it does not have priority in this scenario.

Understanding this helps you remember that left turns always require yielding to oncoming vehicles going straight or turning right, ensuring safe and orderly traffic flow.