Hi @Ben551,
No, it works the other way around.
A host can send a special offer, but a host can only manipulate the host part of the deal. I.e.: Airbnb will automatically calculate the guest service fees, on basis of the amount entered by the host.
Obviously a host can manipulate the host part, in such a way. So it matches the total agreed cost for the trip. I also think, this particular guest paid in a different currency. So, different currency exchange rates will be applied daily.
In other words: The guest will have pick up the bill for currency exchange rate differences. But it can both go down or up. A special offer is only valid for 24 hours.
When I offer a deal (in words), I will always refer to the gross host price (not the guest price, as in essence: we have no control over this).