Please read the following scenario:

Moore contracted in writing to sell her 2010 Hyundai Santa Fe to Hammer for $16,500. Moore agreed to deliver the car on Wednesday, and Hammer promised to pay the $16,500 on the following Friday. On Tuesday, Hammer informed Moore that he would not be buying the car after all. By Friday, Hammer had changed his mind again and tendered $16,500 to Moore. Although Moore had not sold the car to another party, she refused the tender and refused to deliver. Hammer claimed that Moore had breached their contract. Moore contended that Hammer's repudiation had released her from her duty to perform under the contract.

Which law/rule is in play here? Please give a brief definition of this law.Who is correct, and why?