Valentine spotted a lovely blackwood bookcase in an antique shop owned by Tristan, which had a price tag of $5,000. Valentine approaches Tristan and tells him she will take the bookcase for $3,000. Tristan says that isn't possible but how about $4,000? Valentine says no, how about $3,500? Tristan says no to which Valentine replies that she will accept Tristan's offer of $4,000. Tristan does not reply but turns to serve another customer Isolde, as Valentine steps away from the counter to take a call on her mobile phone.

Valentine then turns to pay for the bookcase, but Tristan says he has already sold it to Isolde for $5,000. Valentine is furious and says that she and Tristan had a contract and the bookcase was hers. Is Valentine correct? Give reasons for your answer.