business law subject

Johnny wishes to purchase a new smartphone. He visits BJ Hi-Fi and cheques out the range of smartphones they have available. One particular model, the Orange ePhone, is on sale. There is a small stack of ePhone under a sign that states: 'Special offer! Only $500 !' Johnny says to himself, 'Well, that's an offer that is too good to refuse' and takes one from the stack. He takes the ePhone to the counter, hands it to the cashier, and says 'I'll take this'. He then remembers seeing ePhones on sale for $400 at another store. He says to the cashier, 'I'm sorry, I've changed my mind.' The cashier responds, 'No, I'm sorry, I'm afraid it's too late. You have to buy this now. You are tegally committed.' Is the cashier correct Focus upon whether or not there is an agreement between Johnny and BJ Hi-Fi. Your answer must be supported by relevant legal principles and case/s that you have learned from your readings