You know my h tried to do the same but it was over text instead of IM - kind of the same thing though. I just politely told him that I was happy to have the discussion with him if that was what he wanted but that I was not prepared to do it over text. Maybe you could say something like 'I'm really happy to discuss this but perhaps we could arrange a more convienient time? I am more than happy to talk to you about it over the phone or face-to-face'
