Three things to keep in mind is that the service fee that hosts pay is much lower than the one's guests pay so if you were quoting the price price from the host dashboard it will be different than what the guest pays.
Also Even if both the host and the guest trade in EUR AirBnB will convert the funds to USD to ajudicate the transaction and exchange rates fluctuate.
Finally, if you have smart pricing on, prices will vary.
Of course if you wanted to honor the original price you could send a special offer, but if you do that all of the deficit will come out of the host's pocket. Personally, I'd tell the prospective guest that there is nothing the host will do about the flucuation and if the guest wants a better answer they can call AirBnB directly because hosts are not involved in calculating rates or service charges and you have not changed your rates (if you haven't).