How to value app

I have built and published an app for small business, but had difficulty marketing. I recently had a small business owner approach me and wants to partner with me. He wants to pay some cash up front and then take the responsibility for the Marketing and Business side which is fine by me. My problem is how to value the app/business so far since i dont have any user traffic and most of my investment has been a year of my time in the app development. I built the Frontend, backend and hosting on my own. What would be a fair valuation of a Mobile application for both Android and iOS, Stripe integration, Firebase, Google map etc. I need the fair valuation for the upfront cash calculation Edit: To clarify, he is investing and becoming part owner