Assignee going around assignor after escrow period.

[Craig E.]

Scenario: The signed A-B contract cannot be altered before assigning it to C (absent agreement by all parties, not likely and cumbersome). What happens if C (conveniently) fails to close and we reach the deadline? C loses his right to purchase under the A-B contract. B loses his right to purchase under the A-B contract. A and C now have the right to enter into a new contract for a new agreed upon price. What happens to B and B's assignment fee? How can this easily be prevented?