Ask HN: How to "fire" an app co-developer?

I just launched a technical e-book, and I have been in talks for a month with a developer in the space to put together an app. Recently, I was approached by another - better - app developer in the field, who has a more powerful platform, better design prowess, and better rapport with me. I haven't done much work with the first developer, but he knows the second - I don't want to cause a stink, but I know the second developer will help me make a much better product. Any advice for how to do this?