Any employee in the service, after having made contributions covering a period of 10 or more years to the annuity and benefit fund herein provided for, may elect to pay for and receive credit for all annuity purposes for service theretofore rendered by the employee to the Chicago Transit Authority created by the Metropolitan Transit Authority Act; provided that if the employee has more than 10 years of such service, only the last 10 years of such service shall be credited. Such service credit may be paid for and granted on the same basis and conditions as are applicable in the case of employees who make payment for past service under the provisions of Section 11-221, but on the assumption that the employee’s salary throughout all of his or her service with the Authority was at the rate of the employee’s salary at the date of his or her entrance into the service as an employee. In no event, however, shall such service be credited if the employee has not forfeited and relinquished pension credit for service covering such period under any pension or retirement plan applicable to the Authority and instituted and maintained by the Authority for the benefit of its employees.