Proposing collaboration to an invited speaker?
I don't see any ethical dilemma here unless you threaten to retract his invitation. Don't even hint at it. You liked his work enough to invite him to speak, you should feel free to invite him to collaborate. If he declines, thank him for considering it and tell him you're looking forward to his talk this summer. If he shows interest in collaborating, keep the email conversation going, set up a phone call, etc, etc, and see where it goes. You might be able to use his visit to get some real work done (write a paper, write some code, write a grant, etc) if you're already agreed to collaborate. Don't waste your time between now and then.