He counts against the cap until waived, not renounced. He is a player under contract, not a free agent. The luxury tax is based on the salary of the roster on the final day of the season, not prorated based on how long players on the team, so there is no pressing need to dump him immediately. He will probably be kept until Ainge absolutely needs the roster spot, which will be when rosters have to be cut down to 15, unless Ainge can make a trade that clears some roster space.