The Clifford Chance opinion concludes: "There is no precedent for what should happen to payments to the EU and to the EU's assets on a member state's departure from the EU. The UK and the EU must, to some extent, make it up as they go along, which offers flexibility but also considerable scope for disagreement. Deciding on the relevant principles is anything but easy. Applying those principles is extraordinarily difficult."
The fact that Article 344 TFEU prohibits EU Member States from submitting the legal interpretation of the EU Treaties to a court other than the Court of Justice of the EU may prove to be an obstacle to the creation of any tribunal. Art 344 states - "Member States undertake not to submit a dispute concerning the interpretation or application of the Treaties to any method of settlement other than those provided for therein."
Doubtless, this is a topic to which we shall have to return. Escape from the EU without a substantial payment seems highly unlikely if the UK is to have any meaningful future relationship with the EU.
The House of Lords has done very useful work on many aspects of Brexit. See their Brexit Roundup with links to key reports, analysis and debates on Brexit from the Lords organised by topic.
See also House of Commons Exiting the EU Committee