escape clause 
noun
A contract provision which specifies the conditions under which a party can be freed from an obligation.However, he said the indefinite agreement did have an escape clause in case one party wanted to pursue other avenues....
- The contract has an escape clause that allows him the right to opt out within seven days of Sunday's final game in San Diego.
- Granted, there is an escape clause in the contract but it is limited to a specific injury and for a specific time period.