Be careful....

Even you get a promotion immediately, you most likely will have some problem with your manager later as your manager wouldn't trust you any more as other employees and it is a threat to your manager.  If the offer is good in your opinion, just take it.  Or, just wait for the promotion - after all, your boss promised a promotion to you already and you don't really need use this way to push him if a few months don't really make any difference to you in long term. Certainly, it might work well in your company as different companies have different cultures.   

Just my two-cent and good luck!