Donald is the owner of a recently closed gas station. John, a neighbouring filling station owner, agreed to buy the gasoline remaining in Donald's tanks. Donald tested the gasoline for the presence of water, which would destroy the gasoline, and found it to be "clean." He then called John, informing him of the test results and quoted him a price. John reaffirmed his agreement to buy and arranges pickup of the gas two days later. During the intervening 18-hour period, a large amount of rain fell and with the excess amount left on the ground, water drained into the tank and ruined the gas. Upon hearing this, John refused to buy, but Donald pointed out that the gas was already John's and insisted on payment. Advise John of his rights ( if any). Refer to rule of transfer of title in this case. Define and explain any legal concept used.