The puzzle of that final click

An Internet surfer wants to buy a product online. He clicks on the advertising banner, finds out about the product but does not buy. The next day, he returns to the retail site via Google adwords but still hesitates. It is only the day after that he decides to go ahead with his purchase after having followed a retargeting banner. So, who should be paid for the sale? The last click that closed the deal or the first, the initiator without which the sale would not have been possible?