Brexit: May's EU deal not binding, says David Davis

[IWalkLikeHimaMartian]

If you think about it logically, if the UK actually owed money, that figure would be accountable, exact, and would have been explicitly stated by Barnier, would it not?

If you think about it logically, no-one can possibly know what circumstances/costs, etc will be until the bills/invoices actually arrive.
The bill is for commitments in the future, not in the past.
e.g the exchange rate is/will be an unknown until the bill arrives.

Barnier, nor anyone else can predict the future.
If you ask noseall how much it will cost to build his next project over the next few years, he will only be able to give you an approximation. He might give you a binding quotation, but that is not what is being presented to UK. It is a complicated set of what will be included, and what will be excluded. The cost of which, when it is presented, is as yet, unknown.