Hi, that shouldn't happen if you have definitely set a minimum price on Smart Pricing. You will not be able to change a request to book already received with the lower price. However you should contact Airbnb and report the situation to them so they can check your settings from their end. On the rare occasion if they find that it is a system glitch, if it is not a huge amount they may kindly top up the difference but not guaranteed.
I would suggest you contact them ASAP via telephone so you can go back to the guest with a firm answer to decline, offer increased price or with plenty of luck, accept at current offer if Airbnb do agree to top-up. But basically, like you say, it’s not worth letting the accommodation out if you will be running at a loss!
Click on link for contact options - Contact Airbnb
I hope you get it sorted quickly,
Ama