GreenCar owns a fleet of 10 identical energy-efficient, electric green cars that it rents out for special events. GreenCar is the only company that has such specialized cars available for rental. GreenCar needed all 10 of its cars to fulfill a contract to provide 10 identical green cars to carry dignitaries in the local Earth Day parade on April 22, but each of the cars needed repair to be operable for the parade.

In order to have all 10 cars repaired in time for the parade, GreenCar entered into a contract with RepairCo pursuant to which RepairCo promised to repair all 10 cars and return them to GreenCar no later than April 21 for $1,000 per car, $10,000 total.

On April 21, RepairCo had completed repairs on only 6 of the 10 cars and returned those 6 cars to GreenCar. When RepairCo delivered the 6 cars, it informed GreenCar that the remaining 4 cars were not ready because RepairCo workers had walked off the job when salary negotiations broke down. RepairCo also explained to GreenCar that it planned to give its workers the raises they wanted, but it first wanted to teach them a lesson. RepairCo estimated that the remaining 4 GreenCar cars (all still inoperable) would be repaired by April 30.

GreenCar demanded that RepairCo return the remaining 4 unrepaired cars immediately. RepairCo did so. GreenCar refused to pay RepairCo for any repairs to the other 6 cars.

RepairCo sued GreenCar, alleging that GreenCars refusal to pay anything was a breach of contract. Was it?

If possible, please explain using legal principles and include any positions GreenCar can make in its defense.