(E) Exercise. The term exercise, when used in reference to an option, means the act of acceptance by the holder of the option of the offer to sell contained in the option. In general, the time of exercise is the time when there is a sale or a contract to sell between the corporation and the individual. A promise to pay the exercise price does not constitute an exercise of the option unless the holder of the option is subject to personal liability on such promise. An agreement or undertaking by the service provider to make payments under a stock purchase plan does not constitute the exercise of an option to the extent the payments made remain subject to withdrawal by or refund to the service provider.