And the best workaround you can come up with is to walk away from the money entirely? I don't buy it.

If you go through life acting as if your akrasia is so immutable that you have to walk away from huge wins like this, you're selling yourself short.

Even if you're right about yourself, you can just keep $1000 [edit: make that $3334, so as to have a higher expected value than a sure $500] and give the rest away before you have time to change your mind. Or put the whole million in an irrevocable trust. These aren't even the good ideas; they're just the trivial ones which are better than what you're suggesting.