MtGM,
You say the flaunting the cash and then changing your mind scenario wouldn't happen?
Even if it wouldn't, it still illustrates the position of the law on theft. But I don't think it's so far-fetched.
Suppose a rich person gets very drunk, takes someone home and in that inebriated state promises them loads of money, jewels, etc., all of which are there in the house. They both fall asleep and next morning the rich person, now sober, takes back the offer.   
Or suppose the rich person offers a friend or acquaintance some valuables, but then they have a quarrel and the offer is rescinded.    People DO change their minds about offers they make, sometimes at the drop of a hat.   What matters to the law is whether there was or was not consent at the time of the act in question.

[["If you lead someone on to believe that you want something, are 'gagging' for it, then change your mind at the last moment, chances are, you're going to get a reaction you may not like."]]
Perhaps not, but you still have the legal right to change your mind. If I say you can borrow my car Tuesday, but then I remember that I need to have it that day, or there's some other reason I want to change my mind, I'm legally allowed to -- and if you went ahead and took the car after I'd said "no," you could be charged with theft.
If my change of heart was capricious and inconvenienced you, I might get a reputation for being a selfish jerk, but that's not a legal issue.