What does the girl mean by "haha, let's talk about it later"?

The girl turned you down temporarily. What he means is that if he is not available now, he can't come out to see you. Doesn't mean he will definitely refuse you. You should take the initiative. In case he is free one day, he may come out to see you. You can contact him more and make him feel that you are enthusiastic and trust you. When he is not busy, you will make friends with him, but you are willing to go out for dinner with you.