maxdecatt - the arrangements for the withdrawal of a state from the European Union are set out in Article 218(3) in the Treaty on the Functioning of the EU. They have said on many occasions that they will not renegotiate with the UK. Any politician who states that they can do so is either pig headed, arrogant or stupid (or all three)
As regards the "signing off" of the EU's annual accounts, there appears to be a slight misunderstanding here. Included in their opinion of the 2016 accounts the state " we were therefore able to give a clean opinion of the accounts as we have done since 2007"
Auditors' opinions are given on the accuracy and reliability of the accounts, that they present a true and fair view and follow the rules of financial reporting. If they qualify a set of accounts they give their reasons for the qualification.
The auditors have found significant errors in how the money was paid out. This does not mean that all the people involved were fraudulent. The UK's Public Accounts Committee concluded that for years the complexity of the EU's spending programme, which creates misunderstanding, contributes towards these errors. It doesn't mean that the money was wasted but that it wasn't paid out according to the rules. eg - awarding and EU funded contract directly without holding a proper bidding process .However, the most common type of error is when people claim for costs which they aren't eligible for. Beloved (by some) NF has had his salary halved on order to recoup misspent money.