Cut-Rate Construction Company (CCC) begins building a restaurant for Diners Restaurants, Inc., but after two months demands an extra $100,000. Diners agrees to pay. IF CCC offers, as a reason for the extra $100,000, that ordinary business expenses have increased, the agreement is:
enforceable as an accord and satisfaction
enforceable because of unforeseen difficulties
unenforceable as an illusory promise
unenforceable due to the preexisting duty rule

Unforeseen difficulties can make Cut-Rate Construction Company (CCC) to ask for an extra financial obligation from DIners Restaurant Inc in the process of a contract between the two parties on building a restaurant

Unforeseen difficulties are difficulties that come up in a contract that no party in the contract could have foreseen. However, it becomes the basis on which a further obligation may arise which is enforceable either voluntarily by the parties involved or by a court of law, should it become a litigation issue.

Unforeseen difficulties basically allow the modification of an existing contract based on complications that may arise during the course of the performance of the contract.

In order to modify the existing contract to accommodate the unforeseen difficulties, a novation ( replace the existing with a superseding one) or a Rescission (stopping the contract before it goes further) can be applied.