16 Pa. Stat. § 1791
The sinking fund commission shall annually apply all interest received on sinking fund deposits, and all interest received on bonds held in the sinking fund, and all other income, if any, from the sinking fund, for the purpose of reducing the amount of money required to be paid by the county for sinking fund purposes for the ensuing year, unless such income is necessary for the purpose of having adequate funds on hand to pay the bonds of such county as they mature and become payable. The income so applied, and the amount required to be paid by the county for sinking fund purposes, shall annually equal the full amount required to be paid for sinking fund purposes to the several sinking funds.
The commission shall have the power, whenever it deems it necessary and for the best interest of the several sinking funds, to sell any bonds held by it other than those of the county itself.
16 P.S. § 1791