Bob offered Alice an X amount of money to complete a job. A hash value of the offer was calculate, the offer was digitally signed by Bob and sent to Alice. At pay time, Alice presented a request for the payment of $100, but Bod claimed that the offer was for $50. He presented a copy of the offer with a hashed value that confirms the $50, but Alice presented a different hash value that shows a $100 pay. How would you determine if Bob changed the offer and calculated a matching hash value or Alice change the offer, and calculated a matching hash value?