No, because it is illusory[1], you cannot enter into a contract that basically says "ok you get to dictate the terms and I'll do whatever you say".
That said, I think a lawyer could make a strong claim that later accepting the $20k (presumably he had to provide some sort of payment info for this) gave Mojang at a minimum the rights to use it in the game.
That said, I think a lawyer could make a strong claim that later accepting the $20k (presumably he had to provide some sort of payment info for this) gave Mojang at a minimum the rights to use it in the game.
1: https://en.wikipedia.org/wiki/Illusory_promise