If a Mileage log is incorrect for a user then unsubmitting a claim does not update the mileage log, and as long as it is resubmitted under the same reference this will be corrected at that point. If the claim is deleted rather than resubmitted then the log will not be updated and the best way to address this is to submit a negative value claim, unsubmit it, and then delete it.
For further assistance, please raise a new case online with details of the user, claim date and mileage changes involved.