A win-win agreement presupposes that it is possible for you and your client to come to agreement in which both of you are left better off than you would’ve been without a deal. In my experience there is always a path that leads to a win-win negotiation. You might believe deeply that both you and your client can win. But it doesn’t mean that your client is seeking a win-win agreement. They maybe seeking a win-lose agreement. They may believe in order for them to win you necessarily have to lose.