Wes agrees to install a new hard drive and modem in Mary's computer in exchange for four of her used textbooks. After he installs the hard drive, Wes says he won't install the modem unless Mary gives him two more books. What legal position are the parties in now?

Mary can sue for breach of contract, because Wes had a preexisting duty to do all of the work.

Explanation:

Wes agrees to install a new hard drive and modem in Mary's computer in exchange for four of her used textbooks. But after installing the hard drive, Wes disagrees to install the modem in Mary's computers unless Mary gives two more books to Wes.

To this Mary can file a case against fraud and sue Wes for violating the contract because Wes had mentioned in the contract to install both a new hard drive and a modem in Mary's computers.

Thus the answer is - -

Mary can sue for breach of contract, because Wes had a preexisting duty to do all of the work.