Case study: X, a café owner, agrees to buy 500 kilograms of coffee beans from W, a supplier, for a price of $2000.  W tells X that the beans are packed and ready for delivery.  W says he wants payment to be made before X takes delivery.  X says that he will only pay for the beans after they have been delivered and when he has had time to organise payment from his bank account. X needs the beans urgently because his supplies are running low. W has not supplied coffee beans to X before this occasion.