Y meets Z on 1 July and hands Z a signed written offer (including all the material terms), for the purchase of Z's Rolex watch. Y's offer is for R50 000 and one of the terms of the offer states: "This offer lapses on 30 August". Z accepts the offer and signs this document on 15 August. Z posts the document to Y on 28 August. The letter reaches Y on 1 September and on this date Y opens the letter and notices that Z has signed the document. Did Y and Z conclude a valid contract of sale? Discuss fully