job search - Accepting European math postdoc offers and leaving after a year

I am a finishing Ph.D student in pure mathematics in the US. In November, I received a 3 year postdoc offer in Europe on a PI's grant with a December reply deadline and accepted starting in 2015. Recently, I was offered an NSF MSPRF at an American school, starting in 2014. My plan is to use the NSF in 2014-2015 and 2016-2018 and go to Europe for 2015-2016--the NSF and my US host institution are OK with it. However, when I accepted the European offer I did not specify that I would be staying for only a year (I had no other offers at that point). A what point am I obligated to tell the European PI that I will stay for only a year? Now, before accepting the NSF? After starting the position in 2015? More generally, in Europe is it considered normal or unethical/breach of contract to leave multi year (mathematics) postdoc positions after a year? I know it's considered normal in the US but the postdoc hiring here is done at a departmental rather than individual level...