Is one who buys a fuel voucher designated for government vehicles and uses it for his private car sinful?

It is forbidden to buy stolen, usurped, or embezzled goods if the buyer is aware of their origin, due to the prohibition of cooperating in sin and aggression, and because the sale is invalid as the seller does not own the item being sold. The buyer must return the purchased item to its true owner; if it cannot be found, then its value must be returned. The buyer can demand the price paid from the seller. If the seller refuses, the matter should be taken to court. Otherwise, the buyer's right remains a debt owed by the seller until the Day of Judgment.