Such an agreement would not be a smart move without it being signed in writing. If you want that sort of agreement, you have the framework of a contract extension and should just sign the deal. Realistically the first 3 years are super easy, as the ranges of pay are really narrow. It opens up a bit more in years 4-6, but it still governed by the restrictions of arbitration. Year 7 is where it really opens up as the player is giving up free agency. If they are agreed on year 7, the first 6 years are much easier to define, at that point you literally have an 7 year contract outlined.