Problem: Mary had a contract to paint Peter's fence for $2,000. Mary painted the fence and Peter did not pay Mary (under the contract he was to make payment 7 days after completion of the job). Peter was happy with the paint job; he didn't pay Mary because Peter had lost his job and did not get a paycheck that month. Six months later, Mary needed money badly. Mary told Peter that if Peter would give her $1,100 immediately, Mary would release him from the other $900 owed. Peter agreed and gave Mary a check for $1,100. On the check, Peter wrote "payment in full." Mary cashed the check. Two months after cashing the check, Mary demanded the other $900 from Peter and filed a lawsuit. Who should win and why?