Several things stick out to me. Rezoning mapped floodplain to residential makes no sense. If the owners are not willing to pony up the cash to do a delineation of the floodplain, then they have to accept it as mapped. That would allow the construction of one new house (potentially). If the buyer doesn't like that, too bad.
There are other people willing to purchase the land, but the current buyer has a contract. If she can't build the three homes she wants, she needs to decide to build just one, or maybe none. If she is not willing to accept those terms, she needs to take a hike. That contract to sell should also have certain requirements (such as that the property be rezoned, etc.). If those conditions can't be met, then the buyer has the choice of purchasing it anyway, or dropping it. She can't hold up the sale indefinitely. The last thought is that the contract should have a date by which certain actions must be taken, or the contract is no longer valid.
It isn't really all that big of a problem. The only thing that needs to be dealt with is the buyer.