Mark and His Wife Jean Had Recently Built a Country

Mark and his wife Jean had recently built a country home on a one-acre lot. Initially overwhelmed by the size of the lawn Mark arranged for Stanley, a neighbouring farmer, to cut their lawn with his riding mower. The first cut was difficult and time consuming as there were many tall grasses and weeds to cut through. On this occasion, Stanley charged $300. One week later, Stanley cut the lawn again and charged Mark $200. Reflecting on the substantial expense this represented, Mark thanked Stanley for his services and informed him that he planned to purchase his own lawn mower and take over the job himself. Mark also told Stanley that if he were stuck in the future, he would call Stanley to help him out. Shortly thereafter Mark and Jean took a two-week vacation and on arriving home discovered that Stanley had mowed their lawn in their absence. There was an invoice stuck to their door stating $200 for mowing services and $100 for weed spraying. When questioned about the bill Stanley explained that he understood Mark wanted him to mow the lawn if he were unable to.
What legal issues does this situation raise and what arguments would be used by the parties involved?