In the European Commission's documents on the future of a European contract law freedom of contract is the leading principle of both a CFR and an optional instrument. In this paper, it is argued that an optional instrument as proposed by the European Commission will result in social dumping. However, this can be prevented, if the leading principle of an optional instrument will be a balance between freedom of contract and social justice or fairness. If not, it will be relevant to assess to what extent more stringent national mandatory rules can still be applied if parties have selected an optional instrument to govern their contract. In order to do so, first it is considered whether the optional instrument may be regarded as harmonization, if so, what type of harmonization it fits best. Finally the technique by which an optional instrument may be rendered applicable will be discussed.