What is the legal position in law between Adam and Ben at the time Adam seeks to sell the car to Chipo?

Adam makes a written offer to Ben to sell him a car for $5000 and a radio for $200.he stipulates that the offer must be responded by 1 july 2011.on receipt of the offer Ben telephones Adam found him absent and leaves a message that he could pay only $500 for the radio and he wanted to know the model of the car.on reading the message and well before 1 july 2011 Adam seeks to sell the car to Chipo.|||Adam and Ben are in loco pardooldis.