I think you read into it an ethical dilemma that doesn't exist. You were asked for "intent", not "decision". The school knows the difference, and knows very well to ask for deposit when needed.
I judge "intent" from a "legal" point of view: you may have the intent now, but it may perfectly legitimately change over the course of time - as your circumstances change. You are telling the truth.
I would wait as long as possible - maybe you'll get your other news before the end of the 3-week period - and then state my intent on the last day, or maybe a day before: exactly as they asked for. If and when, afterwards, you get into your dream school - just send them a love letter, citing all the same reasons adcoms cite in our ding letters, something along these lines:
"This year, the pool of the schools I applied to was incredibly competitive. I can assure you that I have no doubt your school would've undoubtedly succeeded and thrived had I chosen to matriculate there. However, as every year, I am compelled to make the unfortunate decision of choosing between some very accomplished MBA-conferring candidates.
Should you re-apply next year to offer me a position within your esteemed institution of higher learning, I can provide you with an extensive feedback on your shortcomings in this year application. While the process of declining an offer from a school is holistic and based on multiple, not solely quantitative factors, I may suggest, for example, that you look into your scholarship offer and / or fellowship opportunities and fee waivers.
I wish you all the best in your educational future, whichever path you should chose - in or out of the Top-10 list".