I was proposed to be promoted under the condition that I promise to not leave my current company within a year. There is no contract, just an informal agreement. My question is whether it is reasonable to request such a promise and how should this informal contract be approached if I receive a competitive offer within this year.
My question is whether it is reasonable to request such a promise and how should this informal contract be approached if I receive a competitive offer within this year.
Yes, it's reasonable to request such an informal promise. Your boss is basically saying "I'd like to promote you, but for it to be worthwhile to the company, I'd like you to promise me that you'll stick around for at least a year."
If you plan to stick around for at least a year, then it's reasonable for you to make such a promise.
Since this is an informal promise, and not contractual, you get to decide how much your word/promise is actually worth.
If you don't care about standing by your commitments, then just accept the promotion, and if something comes up just say "I changed my mind."
If you do care, then don't make a promise you won't keep. If you need to keep open the possibility of leaving within the year, just say "I can't make that promise."
As @HLGEM rightly points out - it never hurts to be known as someone who will keep a promise even when it is difficult!
It's entirely reasonable for a manager to want to invest in someone who's going to be around long enough for that investment to pay itself back.
There's nothing wrong about asking you what your plans are. If you don't know, you can ask for a bit more time to think about it, or you can say you don't know (in which case someone else will probably be given that position) or you can lie about it (which I would consider a bit too selfish, personally).
As you said, all you're being asked for is your plans and expectations, not a signed commitment. If you can honestly say "yes, I plan to stay for at least a year", and then six months from now something major causes you to change your plans... well, life happens.