Just to be clear, if you volunteer, it is what you and the carrier agree to. Despite the suggestion that the EUR 500 offer was a mistake because the EC 261/2004 compensation would have been only EUR 400, there are many occasions when carriers will pay more than they must in order to find volunteers rather than create ill will by denying boarding to passengers who must travel.
Bottom line is that if OP was offered EUR 500 to give up his seat and accepted on that basis, that is what A3 owes him. It has nothing to do with EC 261/2004. Passengers need not be EC 261/2004 experts. When offered EUR 500 to give up one's confirmed seat, the question to ask oneself is not whether that is more than is required under the Regulation, but it is a sufficient amount to act.
Last edited by Often1; Sep 5, 2016 at 4:13 pm
Reason: Missing EUR # for A3 offer