The Commonwealth shall pay the salaries and expense allowances of such sheriffs and their full-time deputies, and of the compensation and expense allowances of their part-time deputies, fixed as provided except that beginning July 1, 1982, such payments to any eligible county or newly formed city under the provisions of Chapter 39 (§ 15.2-3900 et seq.) or Chapter 35 (§ 15.2-3500 et seq.) of this title, which elects to receive state law-enforcement assistance in accordance with the terms of Article 8 (§ 9.1-165) of Chapter 1 of Title 9.1 shall be reduced by an amount equal to the salaries and expense allowances of its law-enforcement deputy sheriffs or the amount of state assistance to be received by the county or newly formed city under the provisions of Chapter 39 (§ 15.2-3900 et seq.) or Chapter 35 (§ 15.2-3500 et seq.) of this title, pursuant to Article 8 (§ 9.1-165) of Chapter 1 of Title 9.1, whichever is the lesser. Such salaries shall be paid in equal monthly installments and the expense allowances shall be paid monthly when the amount thereof is established as hereinabove provided, except that the Board may provide advance payments on a monthly pro rata basis to any county or city and adjust subsequent monthly advances based on actual expenditures incurred in the preceding month. Notwithstanding the provisions of this section, the General Assembly, through the general appropriation act, may allow any locality receiving a 100 percent apportionment of law-enforcement assistance to continue to receive such full apportionments.