Agree with Linda, credentialing/fee negotiation/contracting is not easy, and I have seen what happens when someone tries to do it for a provider when they have no experience with it. The end result was not pretty, to say the least. The biller lost the client half-way through the process because so many mistakes happened.
It is something I would have an experienced credentialer or healthcare attorney handle. While I understand wanting to sign a client, the client has to understand that some things can't be handled by you at this point, and are best done by someone who specializes in it.