Populorum Progressio §59

⁵⁹ The teaching set forth by Our predecessor Leo XIII in Rerum Novarum is still valid today: when two parties are in very unequal positions, their mutual consent alone does not guarantee a fair contract; the rule of free consent remains subservient to the demands of the natural law. (57) In Rerum Novarum t his principle was set down with regard to a just wage for the individual worker; but it should be applied with equal force to contracts made between nations: trade relations can no longer be based solely on the principle of free, unchecked competition, for it very often creates an economic dictatorship. Free trade can be called just only when it conforms to the demands of social justice.