Rollo owns a glove that was worn by Michael Jackson at the First BET Annual Awards presentation. Morant, who is a Michael Jackson fanatic, sent a fax to Rollo on January 1, offering to purchase the glove from Rollo for 2 million dollars. On January 4 Rollo sent a letter by mail to Morant agreeing to sell the glove, but for the price of 3 million dollars. On the same day, January 4, Morant sent a letter by mail to Rollo revoking the offer to buy Michael Jackson's glove.

Identify and explain two legal concepts implicated by this scenario.