The problem is that if A is perfectly rational, in a sense, he can’t make a credible take it or leave it offer
Of course he can—he signs an enforceable contract to pay C $500,001 in the case that he is ever seen to be offering B more than $8191 for the land, and has done with it.
...but when he signs this contract, he may find out that B signed a contract refusing to accept anything less than $450,000 for the land, or else pay some large some to D. If there’s any lag in communication between the two of them, this is an extremely risky strategy.
Of course he can—he signs an enforceable contract to pay C $500,001 in the case that he is ever seen to be offering B more than $8191 for the land, and has done with it.
...but when he signs this contract, he may find out that B signed a contract refusing to accept anything less than $450,000 for the land, or else pay some large some to D. If there’s any lag in communication between the two of them, this is an extremely risky strategy.