Yes, you forfeit the balance, @Nealie0 .
See this Help article: https://www.airbnb.ca/help/article/1352/what-should-i-do-if-my-coupon-code-isnt-working?
I know! You didn't find it when you did a search for "coupon" in the Airbnb Help Centre. When you are looking for something like that, you have to turn to google, which will point you in the right direction in Airbnb Help. The search function within Airbnb Help itself is more than a little frustrating. I read somewhere that they are working on it.
The answer is the fourth point:
"If you don’t use the full amount of a coupon, you won’t receive the difference."
I save mine for when I will be staying a few days. Once in awhile I just book a more expensive place than I normally would. Might as well use it all up 🙂