Mercedes want Bottas, Bottas wants to Mercedes: Why is there no deal ready?

It is no secret that Mercedes are keen to bring back Valtteri Bottas. The German team see the Finn as the ideal third driver behind George Russell and Andrea Kimi Antonelli. For Bottas, who is without a permanent seat in Formula 1 after leaving Sauber, it would also be a return to the team with which he celebrated his greatest successes. So why is the deal taking so long to finalise?