You might try the UN Vienna Convention on International Treaties:Once reference to this "exit fee" can be found in the Treaty of the European Union, I am sure substantive discussion about its magnitude can take place.
Until then, I imagine it will be laughter all round, from one side of this negotiation at least.
A country can't just walk away from obligations it has under a treaty. It has to honour the obligations it has entered into and "settle up" on the way to the exit.Article 26. "PACTA SUNT SERVANDA"
Every treaty in force is binding upon the parties to it and must be performed by
them in good faith.
Article 27. INTERNAL LAW AND OBSERVANCE OF TREATIES
A party may not invoke the provisions of its internal law as justification for its
failure to perform a treaty. This rule is without prejudice to article 46.