Molly and Pat signed a contract providing that "Pat will furnish the correct used business law book for use in Molly's business law class; and in return on August 15, 2011, Molly promises to pay Pat $50 for the book." Molly took the book and planned to pay Pat. Meanwhile, Pat properly assigned the contract Molly had signed to Jack. When Molly went to class, however, she discovered that the book was the incorrect book. When Jack asked Molly for payment, Molly refused. Molly told Jack that the book was useless to her and that she was not paying either him or Pat anything for it. Jack told Molly that he had an enforceable assignment in the form of a negotiable instrument and that he could collect regardless of whether the book was useless. Molly did not believe him. Continuing with her attempt to save money on books, Molly agreed to buy Tim's U.S. history book for $40. She had an oral agreement with Tim that he would give her the book and that she would pay him in three days. This time Molly got the right book. Tim, in writing, properly assigned the right to the $40 payment to Richard. Richard asked Molly for the money. Molly admitted her agreement with Tim but told Richard that she was not going to pay him because he did not have a negotiable instrument. Molly also purchased a communications book from Sam promising in writing to give him in return the next day, to his order, a used DVD player she owned. What is the effect of Molly agreeing to give Sam a DVD in return for the book?