Don't write a business plan.

Instead spend the time you would have spent on the plan putting together a kickass demo, or working on the product.

Having been approached, you're operating from a position of strength already. You should work that to your advantage - if you don't need the money then you can afford to negotiate more on your own terms.

If some documentation is required put together a Powerpoint of 5-10 slides explaining:

- Your product, and why it is unique (and defensible to competition)

- Your team's credentials and experience

- Your market, it's size

- Your competitors (including SWOT analysis)

- Your funding requirements (including salary)

- Your best and worst case scenarios for growth