No refunding bonds shall be issued in a principal amount exceeding that necessary to amortize the principal of and premium, if any, and interest on the bonds to be refunded and pay all expenses reasonably incurred in the issuance of the refunding bonds less the amount then in any sinking, escrow and other funds which are available for the payment of the principal, premium, if any, or interest on the bonds to be refunded.
Va. Code § 15.2-2645