Is it permissible for a gas station owner to agree with truck drivers to provide them with a percentage of the value of their fuel purchases after six months, knowing that the price of fuel is unified in the state?

If the station owner gives a free percentage of petrol after six months as an encouragement to customers, there is no problem with that, as it is a donation. However, if the giving is part of the contract, there is no harm in it, as long as the percentage is known and regulated. But this might necessitate violating the price set by the state, and this could be sinful if the pricing is fair. It is obligatory to adhere to permissible pricing, and it is forbidden to violate it, while it is permissible to circumvent prohibited pricing.