@Cathy212 Yes, in fact it is against Airbnb's Terms of Service to ask guests to pay more when a Reservation Request has been submitted.
Here is the relevant section of the Terms of Service which addresses this:
7.1.2 You are solely responsible for setting a price (including any Taxes if applicable) for your Listing (“Listing Fee”). Once a Guest requests a booking of your Listing, you may not request that the Guest pays a higher price than in the booking request.
You may choose to flag the message from your host by clicking on the small flag icon next to the message asking for more money.
This will alert Airbnb's Trust and Safety team who will take appropriate action with the host.
Airbnb requires that hosts keep their pricing and calendar availability updated at all times so this behaviour is not tolerated.
I am very sorry you have had this experience!