How to deal with categorical entity where one category can be defined and other is "the rest"

I’ll give a small example of my problem. (It’s not the actual use-case)

So I want to fill a slot in a form with the entity in_stock which can have 2 values: available or not_available.

In my nlu file I added my entities that all map to available. Under synonyms I added those entities under available as well:

- synonym: available
  examples: |
    - pear
    - apple
    - banana

Now, when I ask the user what products would you like to check, this works well when the user wants to check available products. When the user enters pear, the slot in_stock gets set to available and my actions work as planned. However, when a user enters a product that’s not available, RASA doesn’t recognize that product as the entity in_stock with value not_available because i didn’t define it.

How can I still set the in_stock slot to not_available when the user enters out of stock items or items i didn’t think of. Rasa doesn’t recognize those as an entity. I thought of making synonyms for not_available but there are so many options.

Use a custom action