Legalities: FIBA and the NBA recognise each other's contracts, so he can't just go to Europe, FIBA would not allow him to play. This is different from Childress, who could go to Greece because he was a RFA, but had no NBA contract.
Thus, AK can't go to Europe for three years unless he agrees to a buyout of his NBA contract. Well, in theory the Jazz could just waive him, but they'd still owe him his full salary, and this would count against their cap. Both figures would be reduced depending on how much his new contract with CSKA pays him (the amount reduced being calculated thus: [CSKA contract amount - NBA Veteran's minimum] / 2). If they buyout his contract, the buyout figure just becomes his new base contract amount and salary cap amount, and would also be adjusted by what CSKA pays. Note that (1) given he would still be owed money by the Jazz, CSKA would not necessarily have to offer him a lot of money to make it worth his while and (2) because what CSKA pays does not reduce the Jazz liability on a dollar-for-dollar basis, AK could earn MORE money by doing this!!
AK could agree to as little as a $0 buyout, which is what Derek Fisher did. However, like Fisher, AK could then sign with whoever he wanted - which means he could do a CJ Bruton and get lost on the way to Europe and sign with another team in the NBA.