Except as otherwise authorized under § 13-201(b) of this subtitle, a unit may not execute or renew a contract for the employment of a contractual employee unless the Secretary issues to the unit a certification that: the employment of the contractual employee is for services that cannot be performed by assignment or hiring of any nontemporary employee; the rate of pay for the contractual employee is comparable to the rate paid to employees in positions that involve comparable duties, responsibilities, experience, and authority; and the services to be performed under the contract encompass functions that: are infrequent; are needed for a limited time; are unusual; or need to be implemented quickly and for which there is no reasonable alternative. The Secretary may issue a certification for any effective period that does not exceed the period of the contract between the unit and the contractual employee.