Carson sold an automobile to Billings, delivered the car and received the full purchase price. Ten days after the sale Billings discovered that Carson had sold similar cars and had made agreements with the buyers to keep the cars in repair for one year free of charge. Billings drove the car to Carson's garage and demanded that Carson make a similar agreement with him. Carson accordingly gave Billings a written agreement stipulating that he would make all necessary repairs for a period of one year free of charge. At the end of nine months the car required overhauling and Billings had the repairs made at a cost of $150. Is Carson liable for said repairs?