International commercial law

You're query is quite complex and fascinating at the same time. Allow me try to illustrate you about it in detail step by step.

Freedom of contract can be defined –or so I propose- as the "the power of natural or legal persons to create, modify, regulate or extinguish property relations by means of acts of private autonomy based on the exchange of declared wills, issued personally or by a representative, which lays down rules of conduct whose content and modalities are primarily determined by the contracting parties themselves.

To Contractual mercantile Autonomy or Freedom you can attribute a double nature: objective and subjective. " Prima facie ", it appears as the generating force of the juridical act; what the juridical act makes be born, though precisely in all that he turns out to be fitted to a more wide system: the juridical Order.

This way, the private autonomy might be qualified very well as a General Legal Principle; "Source of Law" in Spanish Civil Code.

“Most-Favoured-Nation” (“MFN”) treatment — requires Members to accord the most favourable tariff and regulatory treatment given to the pro- duct of any one Member at the time of import or export of “like products” to all other Members.
It is considered to be a bedrock principle of the WTO.

The principle of Most-Favoured-Nation (MFN) treatment.
It is so important that it is the first article of the General Agreement on Tariffs and Trade (GATT), which governs trade in goods. MFN is also a priority in the General Agreement on Trade in Services (GATS) (Article 2) and the Agreement on Trade-Related Aspects of Intellectual Property Rights (TRIPS) (Article 4), although in each agreement the principle is handled slightly differently. Together, those three agreements cover all three main areas of trade handled by the WTO.

Hope it Helps

Regards,
Kevin M Paul