A year later, that buyer tries to sell that home and only gets 400k for it, then, by definition, that is the market price of that home for that sale. Market price is what someone is willing to give you for a property at the time of sale. Hint, stocks work the same way.
Now, if I have something to offer someone, for example, I am a Supreme Court Justice, and someone that often has financial interest on what is before the court, offers me well more than I would get from any other buyer and does so because they think I will reward them in other ways, then that is different. However, if you overpay for something I am selling without any expectation of receiving any special favors in return from me, then that is just a bad financial decision on your part.