Cats in a sack comes to mind, like cats in a sack

Or if you prefer your analogies more Northern-style, too may ferrets in that trouser leg:

The EU’s chief Brexit negotiator, Michel Barnier, fears the refusal of member states to soften their demands over the size of Britain’s “divorce bill” could lead to a collapse in talks and the UK crashing out of the EU without a deal, minutes of a meeting of the European commission reveal.

Barnier has told the commission president, Jean-Claude Juncker, and other senior officials that the stakes are so high because Berlin and Paris are refusing to pay more to cover the UK’s departure, while those governments who receive the most from EU funds are opposed to any cuts in spending.

And thus those demands for €60 billion, €100 billion, that are being bandied about. Not because there is any legal right to have such sums, nor even a nod to the lads agreement that they would be paid, but because that's the amount that wouldn't inconvenience the others as we leave.

Which isn't, if we're honest about it, that great a justification for our having to open up the chequebook. And it's not as if the EU doesn't have form here, they're prone to backcasting.

The very centre of the Greek debt problem is that the IMF insists that Greece can only manage a primary surplus of 1.5% of GDP, thus can only pay back less debt than is owed. The Eurogroup has been working the other way around, in order to pay back the total that surplus should be 3.5% over decades. Therefore Greece must be plunged into the sort of austerity that no democratic polity has ever managed over the long term just because that's what makes the EU sums add up.

It's the same here. We desire this much money, that's the demand. Not anything to do with how much is owed, due, or can or will be paid.

And we would note one more interesting little point. If it's €100 billion to leave then what would have been the even greater costs of staying? For this demand really does only take us up to 2020......