Members of Council, members of the Civil Service Commission, members of the City Planning Commission, members of the boards or commissions of the City which do not involve full-time employment and any officers or employees in the administrative service of the City who are authorized by the Mayor to accept compensation or salary from other sources or authorities, and officers or employees in the administrative service who by the terms of their employment are permitted to engage in employment outside the City service, may receive salaries or compensation for such outside employment. No officer or employee in the administrative service, the conditions of whose employment require his or her full attention and time, shall accept any other employment than that of the City, nor any salary or compensation for other employment, so long as he or she continues in the service of the City, except as approved by the Mayor in writing. Any such outside employment or the acceptance of any salary or compensation therefor shall forfeit the right of such person to continue in the employment of the City. This section shall not be construed to prohibit the acceptance by any officer or employee of the City, of any pension or annuity to which he or she may be otherwise entitled from any source whatsoever.