you have to have the arb screen in one window and the long term screen in another. then you arb him in one screen and offer the long term in the other screen. Since you open the LT window before he is arb'ed, the logic on that screen doesn't know he has been signed yet. So when you sign him, it loads it into the next open 5 seasons...poof...6 year deal. If you arb him, then open the window, the underlying logic of the LT screen knows he has been arbed a 3rd time already.
It needs to be
(1) Open Arb window
(2) open separate LT window
(3) offer arb in arb window, don't close
(4) switch to LT window, offer LT
(5) celebrate