A contract only comes into existence when BOTH an offer AND an acceptance have been made. As I understand the wording of your question, there's solely an 'invitation to treat' involved, which the person making the invitation is at liberty to withdraw at any time.
It's roughly akin to a shop owner putting something in his window for sale at £x but, when you go in to buy it, then refusing to sell it to you for that amount, or at all. He/she is perfectly entitled to do so as he/she has never accepted your offer to purchase the item, meaning that no contract has been created between the two of you.