"As soon as the applicant obtains permanent alternative employment paying the same or more than his pre-dismissal earnings, his loss attributable to the action taken by the respondent employer ceases. It cannot be revived if he then loses that employment either through his own action or that of his new employer. Neither can the [respondent employer] rely on the employee's increased earnings to reduce the loss sustained prior to his taking the new employment. The chain of causation has been broken."