escape clause

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A clause in a contract that specifies the conditions under which the promisor is relieved of liability for failure to meet the terms of the contract.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun figuratively Any clause, term or condition in a contract that allows a party to that contract to avoid having to perform the contract or cancel it.