Mulligan. In golf, this is “a free shot sometimes given a golfer in informal play when the previous shot was poorly played.” Merriam-Webster dates it to 1931.
Do-over. This is “a new attempt or opportunity to do something after a previous attempt has been unsuccessful or unsatisfactory.” Merriam-Webster dates it to 1912.
Plan B. This is “an alternative plan of action for use if the original plan should fail.”
Exit strategy. This is “a plan for ending involvement.”
Escape clause. This is “a part of a contract that allows a person to get out of the contract in a particular situation.”