Is my participation in helping an employee defraud his company, by inflating ticket prices and then benefiting from the difference, a sin that requires expiation, especially after I admitted that this happened while minimizing the amount out of fear for him of punishment?

You bear no sin for what you did with the manager; you advised him of three ways to deal with the employee's fraud, and he refused, which is sufficient indication of his dishonesty. You are not obliged to disclose the amount of the money, as the manager can find it out through other means. The fraudulent employee must be advised against his actions, and it is not permissible to aid him in his fraud and in consuming ill-gotten wealth.