cooper, the general manager of a courier business, is a holiday in Sydney. he ran out of money. he telephones bob, the most knowledgeable, experienced and trusted courier. cooper asks bob to sell an old computer for at least $600 and deposit the money to cooper's bank account. the next day, cooper was surprised to find that only $250 was deposited into his account. he telephones bob. bob is not answering. bob calls back later and explains that he sold his best friend and could not 'rip him off' by charging $600. cooper is furious. advice cooper if bob is liable for the balance of the money and if so on what ground?