"does the Lisbon Treaty say what we should do regarding financial commitments on leaving?"

No it does not, there is no instrument, or article contained in the treaty that can compel any member state to pay anything - the terms under which a member state leaves MUST BE AGREED by the EU and the state that is leaving. There is a stipulated time limit given to reach that agreement which is within 24 months of Article 50 being triggered by the member state - once that time period has elapsed the state is simply no longer a member of the EU. In the case of the UK that date is the 29th March, 2019.

So far it would appear that while the UK's negotiating team has been waiting to discuss and negotiate all aspects of the leaving process, those in charge of negotiations from the EU's side have not. All we have heard out of them have been threats and demands. They have given a figure which they dictate must be met before negotiations on anything else take place. That figure has not been broken down, it has not been explained, it has not been justified. Neither the UK, or it's population, are a country, or a people, that responds well to being dictated to (France and Germany should know that from history - Napoleon once tried to impose a ban on British trade with Europe - it cost him his Empire). Messrs Juncker, Barnier and the rest that comprise the unelected EU Commission have up to now arrogantly tried to run the clock down in order to put pressure on the UK - If they continue to do so, if they refuse to negotiate all aspects of the withdrawal process then they are going to end up with nothing. Should they then attempt to "punish" the UK with tariffs and trade barriers, they will find themselves in breach and in dispute with the WTO on all five guiding rules of the WTO which all member states of the EU agreed to when they joined, as individuals, on 1st January 1995.