This hadeeth comes under the following chapter: BOTH THE BUYERS AND THE SELLERS HAVE THE OPTION TO WITHDRAW THE TRANSACTION BEFORE LEAVING THE MEETING (WHERE THE BARGAIN IS STRUCK)
Ibn Umar reported Allaah's Messenger (sallAllaahu alayhi wa sallam) as saying: There is no transaction between two persons entering a transaction until they separate, but only when there is an option to annul it.