we once used the radisson price promise successfully at a country inn in canada in 2007. radisson granted a 25% discount from the competing rate. from the information that u've given, one difference that i can see is that the third party website rate is in a different currency. try using a version of that website which is based in a country that uses the euro. it might not matter, but it removes one potential source of argument.
http://www.radisson.com/lrg/home.do
i also once had a price promise on a holiday inn rejected because the rate conditions were different (the competing lower rate did not provide instant confirmation online).
good luck!
ian.
