If this is the case, then he was in fact intending to use that money to break the law. Working to get around the system to avoid paying taxes on that amount of money used in a purchase is against the law. So even though he told them he was going to use the money to purchase a truck, they also know that the intention behind full cash is to avoid the tax system. That is probably why they feel they can get away with keeping his money, because in the end the money was going to be used in the commission of a crime, though that crime would be a federal one.
*Oh, also state and perhaps city too since they can tax on sales too.