"An orderly withdrawal of the United Kingdom from the Union requires settling the financial obligations undertaken before the withdrawal date," said the European Commission document seen by AFP.
"The agreement should define the precise way in which these obligations will be calculated ... the obligations should be defined in euro," it added.
The document did not say how much the Brexit settlement might cost but EU officials have previously said it could be as much as 60 billion euros, sparking howls of outrage in London which puts the figure nearer 20 billion.
The document, titled "Non Paper on key elements likely to feature in the draft negotiating directives," was drawn up for the European Commission which will conduct the Brexit negotiations with Britain.
It covers in more detail the same ground outlined last month by EU president Donald Tusk in response to Prime Minister Theresa May's official March 29 notification that Britain was leaving the bloc.
Tusk stressed then that the EU will insist on agreeing the future of EU citizens in Britain and the Brexit bill first before considering London's demand for a free trade pact.
The two-year negotiations are expected to be very difficult as the two sides try to undo the mass of legislation agreed since Britain joined then European Community in 1973.
Anticipating possible disputes, the draft paper envisaged setting up "an institutional structure to ensure an effective enforcement of the commitments under the agreement," while maintaining the primacy of the European Court of Justice.
For disputes outside EU law, "an alternative dispute settlement should only be envisaged if it offers equivalent guarantees of independence and impartiality as the ECJ," it added.