What is the ruling on the financial increase granted by the state to citizens as compensation for the delay in providing them with cars? Is it obligatory to return it if it is unlawful, and is one who spent it before knowing of its unlawfulness sinful?

There is no blame upon you for taking the additional money from the government and benefiting from it. It is a gift and a grant, compensating for their delay in purchasing the promised car, and it is not forbidden usury (riba) for two reasons: First, the money paid to the government was not a loan. Second, the forbidden increase in loans is that which is stipulated at the time of the contract. As for an increase paid without a prior condition, there is no harm in it.