You shouldn't let others change a contract.
Fred has a contract with his brother George about who does what chores that is being discussed.
Fred wants to finish the discussion.
Fred doesn't allow George to change the contract, insisting they need to stick by it.
Fred lets George change the contract as long as they both agree to the changes.
Fred and George abide by the original contract and get used to silently following it.
Effectively this leads to a new contract discussion every time one of them doesn't feel like doing a chore.