Services can talk to other services. i.e. In your example, the order service can speak to the inventory service to change the stock level. The 'API controller' (this is also a service!) can speak to the inventory service to get the current inventory - but it shouldn't be responsible for handling the transactions of other services.

I think the premise of using a monolithic service (your API "Controller") to handle transactions that it simply shouldn't be concerned with is the main problem here. i.e the problem is that the transaction should not be distributed, not that it is handled through multiple services.

I'm further confused at the reconciliation that seems to occur later in your example. Why is it bypassing the services and writing directly to the service DB itself?