Proposing collaboration to an invited speaker?

I am organising a conference. This will take place in 6 months time. One of the invited speakers works on a topic that is highly related to my research interests. I would like to propose a specific collaboration to this professor, but I feel that the fact that I invited him to the conference (I am paying for his accommodation, expenses ...) may induce some pressure to accept my invitation to collaborate.

I wonder whether I should just wait after the conference to tell him about this project or if it is OK to tell him now? The reason I want to tell him now is that I would not like to wait for another 6 months to start this project.