Hi @a.pinzon,
Related to your post here: Multiple users for Machine to Machine applications - #3 by a.pinzon there might be a couple of ways to do this.
You can add properties to an id token for a user via Auth0 Actions: Auth0 Actions. During the login flow, you can validate the properties on the id token.
Alternatively, if your customers are requesting an access token to then hit your API, you could add scopes (permissions) with required metadata to the token to verify the correct merchant: Scopes.