What can I do if a proposed job promotion falls through?
Most of the time, if your employer does not follow-through on a promised promotion, there is nothing you can do about it. Your only recourse if you don't like your job or what your employer is doing (or not doing) is to quit.
Talk is cheap, and just like with a teenager's promise to do better in school or your significant other's promise to wash the dishes, it depends on the willingness of the other person to carry through on what they promised. The only kind of promises you can count on being performed are those embodied in a contract. Non-contractual promises are not enforceable in court. So most of the time, if your employer does not follow-through on a promised promotion, there is nothing you can do about it.
There are two important concepts to bear in mind about promised promotions or raises. The first is "employment at will." Except to the extent changed by a written employment contract for a set or given period of time (e.g., a one-year contract), all employment in this country is "employment at will." This has many consequences, such as the fact your employer can terminate you at will, without cause and without warning, or that you can equally quit at will with no prior notice.
Basically, the terms and conditions of your employment--your title, your duties, your level or authority, and your pay and other compensation--are 100% under the control of your employer. Your employer can change any or all of them at will. Your employer can therefore also go back on its promises or proposals. Your only recourse if you don't like your job or what your employment is doing (or not doing) with it is to quit.
The second important concept is that of contracts. A contract can modify the above, since a valid contract locks the parties into the rights and obligations contained in the contract. Whatever a party (e.g., your employer) contracted to do (for example, give you a promotion) must be done in accordance with the terms of the contract. So if you had a contract guarantying you a promotion, you would have a legally enforceable right to the promotion, and could sue your employer if you did not get it.
However, only an actual contract, and not any other promise or statement, creates enforceable rights. So the critical question becomes: were the requirements for a contract met? If they were not, the promise or proposal is not enforceable.
What makes for an enforceable contract? Two things: mutual agreement and consideration.
(1) Mutual agreement is fairly self explanatory. The two parties to a contract must mutually or jointly agree to something. Mutual agreement, however, also requires that all key or critical terms of the agreement be spelled out, so that the parties agree to the when/where/what/how/etc. of what should be done. So, for example, an agreement to give you a promotion in six months, or after your next satisfactory performance review, or on the anniversary of your hiring, or after you hit certain targets or accomplish certain, defined things, can form the basis for a contract, since the “when you would get the promotion” is clearly spelled out.
But an open-ended agreement to give you a promotion in the future, without any way of determining the “when” that will be or what triggers the promotion, is too vague as to create an enforceable contract. There is simply no way of knowing if or when you will get the promotion.
(2) There must also be an exchange of "consideration," or things or promises of value. You are clearly getting something of value: a promotion. But what is your employer getting? Unless they are getting something, too, they are not receiving anything of value and therefore would not be receiving consideration. If they are not receiving consideration, there is no enforceable contract.
Furthermore, any consideration must be something they would not be able to get anyway. If they could get it without offering you a promotion in exchange, there again is no consideration. So just working harder or longer hours? That's not consideration: your employer can give you as much work as it likes, and have you work any hours it likes, without having to promise you a promotion.
It would take something like:
(a) agreeing to relocate to a different city (since while they can fire you if you refuse to go, they can't actually make you move without your agreement) or,
(b) turning down another job offer you were about to take and agreeing to stay on
to furnish consideration.
As you can see, most of the time, you will not actually have a valid, enforceable contract for a promotion. If you do not, your employer may legally go back on its promise and not give you the promotion.