Thats the part where an employee should do what employers do all day every day like breathing: lie your ass off.

"I recall the verbal agreement being for 42k, just as it is in the contract. Stop lying."

Probably should look for other work once the error found out though regardless of how you handle it. They'll invent a reason to fire you for failing to be a good lil wage slave, cowering and signing an hr document obligating you to pay the difference back. A month later it will be some "not a team player" dismissal.

Fortunately though, it might never be discovered. Corporate malice is matched only by corporate incompetence.