There is general agreement that there is no legal requirement for the UK to pay the EU anything on exit. There is no provision in the Treaty for an exit bill. We received no payment on joining to deal with liabilities the other members had already incurred. No one suggests if a net recipient country wanted to leave they would receive a leaving bonus. No one in the referendum campaign said we would face a bill.The EU has never produced a legal base for a divorce bill.
Some seem to think we should nonetheless pay something to get a deal. It is most important that this is always called an ex gratia payment or gift, as the UK must not by its language and promises create some legal obligation under EU law that does not exist at the moment. There is a danger in seeking some signed promise of trade talks in return for some written offer of money in turn. Under EU law we could create a legal obligation if we use the wrong words where there is none at the moment. There is a danger that making a contingent offer subject to getting a good Agreement will just be banked by the EU.
Trying to keep discussion of payments to general indications and headings will not stop the EU putting a figure on any indication and applying moral pressure on us to pay that amount. Nor would it stop them trying to get that money without a good Agreement.
As we dont owe anything the best thing to do is to offer nothing. The public show in polls that they have no wish to pay large sums to the EU. It is time to spend the money at home instead.