THESE are exhilarating times for the 52% of British voters who last summer opted to leave the European Union. After months of rumours that an anti-Brexit counter-revolution was being plotted by the Europhile establishment (who even won a Supreme Court case forbidding the government from triggering Brexit without Parliament’s permission), it at last looks as if independence beckons. This week the House of Commons voted to approve the process of withdrawal. The prime minister, Theresa May, will invoke Article 50 of the EU treaty next month, beginning a two-year countdown to freedom.

But the triumphant mood is about to sour, for a reason few people have grasped. The first item on the agenda in Brussels, where divorce terms are to be thrashed out, will be a large demand for cash. To Britons who voted to leave the EU because they were told it would save them £350m ($440m) a week, this will come as a shock. The mooted bill is huge—some in Brussels talk of €60bn ($64bn), enough to host the London Olympics five times over—and its calculations open to endless argument. Until now the Brexit debate has focused on grander matters, such as the future of the €600bn-a-year trading relationship between Britain and the EU. Yet a row over the exit payment could derail the talks in their earliest stages.

The tab is eye-watering. Britain’s liabilities include contributions to the EU’s pension scheme, which is generous and entirely unfunded. The biggest item, which Britain will surely challenge, is the country’s share of responsibility for a multi-billion-euro collection of future projects to which the EU has committed itself but not yet allocated a budget. These liabilities, and sundry smaller ones, may be offset a little by Britain’s share of the EU’s assets, mostly property in Brussels and elsewhere around the world. By one analysis (see article), the bill could be as little as €25bn or as much as €73bn.

So there is plenty to haggle over. But the very idea that the charge is something to be negotiated irritates many Eurocrats, who see it as a straightforward account to be settled. The European Commission’s negotiators insist that the divorce agreement must be signed off before the wrangling can begin on anything else, such as future trading relations. Britain would prefer to tally up the bill in parallel with talks on other matters, in order to trade more cash for better access.

Garçon! This isn’t what I ordered

It is in everyone’s interests to reach an agreement. If talks fail and Britain walks out without paying, the EU will be left with a big hole in its spending plans. Net contributors, chiefly Germany and France, would face higher payments and net recipients would see their benefits cut. For Britain the satisfaction at having fled without paying would evaporate amid rancid relations with the continent, wrecking prospects of a trade deal; a rupture in everything from intelligence-sharing to joint scientific research; and, perhaps, a visit from the bailiffs of the International Court of Justice. Such an outcome would be bad for the EU but it would be even worse for Britain.

That imbalance will become a theme of the Article 50 negotiations. It suggests that the British will have to do most of the compromising. Mrs May must not waste the two-year timetable haggling over a few billion, when trade worth vastly more hangs in the balance. The EU can help by agreeing to discuss the post-Brexit settlement in parallel with the debate about money. Rolling the lot into one would increase the opportunities for trade-offs that benefit both sides.

But there is a danger of hardliners in London and Brussels making compromise impossible. Some in the European Commission are too eager to make a cautionary tale of Britain’s exit. And they overestimate Mrs May’s ability to sell a hard deal at home. The British public is unprepared for the exit charge, which is not mentioned in the government’s white paper on the talks. The pro-Brexit press, still giddy from its unexpected victory last summer, will focus both on the shockingly large total and also on the details (here’s one: the average Eurocrat’s pension is double Britain’s average household income). It has flattered Mrs May with comparisons to Margaret Thatcher, who wrung a celebrated rebate out of the EU in 1984. A small band of Brexiteer MPs have a Trumpian desire to carry out not just a hard Brexit but an invigoratingly disruptive one. Mrs May’s working majority in Parliament is only 16.

Everyone would be worse off if the Article 50 talks foundered. Yet the breadth of the gap in expectations between the EU and Britain, and the lack of time in which to bridge it, mean that such an act of mutual self-harm is dangerously possible.