Travel agency is demanding a $2000 surcharge

I'm in the US and my wife and I booked a very expensive once-in-a-lifetime type trip with a European travel agency for next year. AFTER I paid the full amount they demanded $2000 more due to the rising value of the Euro, which I would rather not pay. They offered a refund minus a $5000 deposit. I know it was a deposit, but I feel like they broke the terms of the agreement by changing the price.

Are they within bounds to keep the deposit? And if not, do I stand a chance of forcing them to return it? At this point I'd rather pay a lawyer than let them have it.