Section 8A. The board of officers who have made a taking under this chapter shall, prior to the expiration of sixty days subsequent to the recording of the order of taking, offer in writing to every person entitled to damages on account of such taking a reasonable amount which such board is willing to pay, either in settlement under section thirty-nine of all damages for such taking with interest thereon, together with taxable costs, if any, or as a payment pro tanto. If such person elects to accept the offer as a pro tanto payment, such election shall be without prejudice to or waiver or surrender of any right to claim a larger sum by proceeding before an appropriate tribunal. If such tribunal shall assess damages in an amount less than the amount actually paid as a pro tanto payment, the petitioner shall be ordered by a proper decree to refund to the body politic or corporate an amount equal to the difference between the pro tanto payment and the amount of damages so found by such tribunal, plus costs and interest at the rate calculated pursuant to the provisions of section thirty-seven from the date when such damages were assessed.

After a pro tanto payment has been made or after an offer of payment has been made in writing as required by this section and not accepted, no interest shall be recovered except upon such amount of damages as shall upon final adjudication be in excess of said payment or in excess of the written offer of payment as herein described; provided, however, that all taxable costs accruing subsequently to such payment or to such written offer of payment shall be recoverable by the petitioner if the adjudicated damages exceed such payment or such offer, otherwise the petitioner shall be liable for and pay such costs.