Is "the contract is the law of the contracting parties" a legitimate legal maxim in rulings on transactions, or is it merely a legal principle?

"Contracts are the law of the contracting parties," unless they violate Islamic law, as Allah Almighty says: "O you who have believed, fulfill [all] contracts," and as the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions." If the contract or condition violates Islamic law, then it is void, even if both parties agree to it, because mutual consent does not make the unlawful lawful, and "every condition that is not in the Book of Allah is void." So if the contracting parties agree to an impermissible condition, it is void, such as the condition of guarantee on the mudarib (investor) in a mudaraba contract; because he is a trustee and is not liable except in cases of transgression.