On 1^st October Ali wrote a letter to Sami offering to sell a quantity of crude palm oil at RM1,500 per ton. Afterwards, Ali realized that the crude palm oil can be sold at a higher price to a third party. On 7^th October Ali wrote a letter revoking the offer. Meanwhile, on 8^th October Sami has accepted the offer through a letter posted at 3 pm. Ali's revocation letter reached Sami at 10^th October. Sami's acceptance letter reached Ali at 11^th October. Discuss whether there is a valid contract between Ali and Sami.