None of the teams that Butler wants to go to will have enough cap space to sign him in the off season. Only the Heat will be able to offer him any kind of real money. Riley is exerting that leverage. Essentially saying to Butler and his agent that this is not going to work. That MIA has the leverage here. If you want to go to HOU for the Min or for the MLE or something, fine, but I am not going to trade you now so some other team can pay you in the off season with Bird Rights.
I think the realization for MIA is that even if they were willing to trade him, what can they get? Trades are really hard these days, especially if it is a contender trying to take in a player (assuming most contenders are over the cap, over the tax, and in many cases, second apron tax).