(a) If the proceedings include any acquisition and the actual cost of the acquisition as finally determined is less than the amount included in the assessment as the cost of the acquisition, the excess may be spent as the legislative body may thereafter determine, either for the maintenance or repair of the work or improvement, or the excess shall be refunded or credited in proportion to the amount of the assessments that were levied for the acquisition cost, as follows:

1. (1) If the assessment and all installments thereof and all interest and penalties due thereon have been paid, the refund shall be returned in cash to the person who paid the corresponding assessment or installment, upon furnishing satisfactory evidence of the payment.
2. (2) If the assessment or any installment thereof is unpaid, the credit shall be applied upon the assessment or upon the earliest unpaid installment of principal and interest.