Dear
This is in reference to the offer letter received from your company. I would like to send thank you for sending me the information on (name of venture)
No doubt the concept is very fascinating and interesting, however I am little reluctant to become involved in any venture that would require my absence from the firm for an extended period of time.
I am sorry but I would be returning your material under separate cover and wish you the best of luck in your new venture.
I hope you find someone soon to get involved in this exciting venture. Good luck!