Contract law imposes the agreeing parties to do their duties and to act reasonably with each other in accordance with their agreement. For example, a contract says that party A is selling their car in excellent working condition and will be allowing party B to pay every 15th of the month for 2 years. Party B is expected to pay every 15th in return to be buying an excellent running car.
Tort law, on the other hand, both parties should act reasonably with each other and they should be liable in their actions. If party B secretly scratched the bumper of the car in the first month of their ownership to get a discount, telling party A it was there since day 1 and had just noticed it, it will be taken to the court to award the compensation to the real victim.