If you delete the rest quantity for an operation, the remaining loading for the operation will also be deleted. The system will also adjust the planned quantity for all the following operations, to the quantity that was reported as finished for that operation.
If you have checked the setting Automatic material reporting, you can also remove material reservations for the deleted quantity by first checking the Delete rest material option. This can also be done using the setting Delete Rest Material During Automatic Reporting?, under the heading Orders under the Manufacturing tab in the Settings procedure.