Lawrence went to Handy Bookshop to buy a tax law book. He told Linda, the shop assistant: "Please give me a copy of Chan's Tax Law". This tax law book was very similar in size and cover design to another book, "Chima on Contract Law". Chan's Tax Law and Chima on Contract Law were stocked together on the same shelf of Handy Bookshop. Linda by mistake picked Chima on Contract Law and sold it to Lawrence for HKD500 believing that she was selling Chan's Tax Law. Lawrence did not realise that he has bought a wrong book until he returned to his hostel. 

Is there a valid contract between Lawrence and Linda?

Will  advice be different if:

 

(1) Lawrence knew that Linda was making a mistake in selling him Chima on Contract Law believing it to be Chan's Tax Law. But he still bought the book without saying a word because the former book was worth HKD800.

(2) Linda thought that Lawrence was asking for "Chen's Toxicology" and she sold him a copy of this book despite noticing from Lawrence's student ID that he was a law student. Lawrence did not notice the mistake until after three months when he opened the book to prepare for exam.

(3) Neither Lawrence nor Linda knew that the tax law had just been changed radically, thus making Chan's Tax Law book totally useless.