If a service on a reservation item has been calculated automatically then no price change is possible. In order to reduce the price you can use the workaround and add a discount in the same way as you would add any other ad-hoc service.

NOTE: If a service has been added to a reservation as an ad-hoc service, then it is possible to change the price while editing reservation item. By choosing Switch to ad-hoc mode option you can see the list of all services for one item and manually change prices and suppliers or delete services.