Charles offered to shovel snow from Wilma’s driveway after she sprained her ankle late in January. For the remainder of January and throughout February, true to his word, Charles shoveled her driveway approximately 5 times. On March 1, when Wilma’s ankle had sufficiently healed and she was able to look after her driveway again, Wilma told Charles how thankful she was for his efforts and that she would give him $20 for each time he shoveled her driveway (or a total of $100). Charles accepted Wilma's offer, but later Wilma refused to pay the money. Can Charles force Wilma to pay?