As long as Manchester City reach their Champions League goal they wouldn't mind him leaving after 2-3 years. Otherwise they wouldn't be very willing to do this I believe.
Manchester City is not going to let him go. At least in two or three years in my opinion. And the way he is playing in Manchester city I am not sure why even he would want to leave. But I can see a good business being made for Manchester city if they are willing to sell Haaland. I am quite sure that they will be getting the 200 million euro release clause for Haaland soon. But before that, he does have two years with Manchester City. And during that period I believe Manchester city will be able to win the Champions League with him.