When an entity, as the term is used in § 10.1-645, agrees to purchase a facility and the rights incident thereto resulting from the storing of additional water in feasible flood prevention sites or the strengthening of foundations and appurtenances of feasible flood prevention sites in which the Board has invested pursuant to § 10.1-638 B, the purchase price shall be the total expenditure from the revolving fund by the Board for such facility plus a surcharge of three percent annually on all funds expended for the facility, other than funds expended pursuant to § 10.1-638 C, from the date of expenditure to the date of purchase by the purchasing entity. With the approval of the Board, the purchasing entity may finance the purchase price, or any portion thereof, of the facility under the terms and conditions of §§ 10.1-638 A and 10.1-643, and the provisions of §§ 10.1-643 and 10.1-644 shall apply, mutatis mutandis, to such financing. If a purchasing entity finances the purchase of a facility as hereinabove provided, such purchasing entity shall not be precluded from applying for a loan authorized by § 10.1-638 A to the limit imposed by that section to complete any facility purchased to store additional water.