Later'' is used to tell WHEN something will happen. 'For a while' is used to tell HOW LONG something will last. 'After a while' means 'later', when something will occur soon. The concert will start later and will go on for a while. = The concert will start in the future and will last for some time (a while). The concert will start after a while = The concert will start later.

if you busy now we can talk later .but if you free can we talk now for while