In this section: “supplement” means a payment in addition to a retirement allowance that is paid to a retiree or a surviving spouse of a member, former member, or retiree; and “supplement” includes a salary, bonus, pension, or spouse’s benefits. Except as provided in subsection (c) of this section, a county may not pay a supplement to a retiree or a surviving spouse of a member, former member, or retiree. If a retiree was covered by the contributory plan of the Judges’ Retirement System as a member or former member on or before June 30, 1974, a county may pay a supplement to the retiree or surviving spouse of the retiree, if: the sum of the supplement and the retirement allowance of the retiree does not exceed $20,000; or the sum of the supplement and the retirement allowance of the surviving spouse does not exceed $10,000.