I'm sure he couldn't possibly want to cause BTC to lose value temporarily so interested party's could cash in on it's value falling when it rises again. Oh I'm such cynic!
Sounds like you just need to wait until the wallet is synced up to the network. Just leave it open and eventually it'll get up to date (it may take many days) but at 2 BTC it'll be worth it. Hopefully you'll be in a position to withdraw it
Sorry to hear about your issue, they're probably not used to receiving such large donations so perhaps there's a chance you could just ask them to return it to you? If it's an error and if they're reputable then it shouldn't be an issue. A donation is a financial transaction at the end of the day ...