I only know from my old job the UK way of working. Yes they can confiscate cash if they don't believe your "reason" for carrying it. They would have to give you a receipt and you could appeal to the Commissioners of HM Customs, an independent body.
We used to get reports of cash over £5K confiscated as part of my work looking into tax evasion and it was amazing the amount of cash folk would give up and never claim back. Guess they had a lot more!
You should certainly appeal as you seem to have a "good reason" for carrying that sort of money, i.e a cash Poker player.
There are various versions of Money Laundering and Proceeds of Crime laws throughout Europe, so it is a minefield. It is not only your reason for carrying the cash, but you may have to prove how you got it in the first place. You should always keep your currency purchase receipt. Nowadays the authorities have carte blanche to do what they like. I got back from Amsterdam today and not only was passport control swiping passports but there was 6 police officers asking questions to random?? travellers. E.g. why were you in Amsterdam? I know we live in troubled times, but I do think it is a bit much to be interrogated when travelling between two countries that in theory don't have any borders.
Very sorry to hear about your problem plplaya - I hope you get your cash back.