Mass. Gen. Laws ch. 79 § 39
Whenever damages may be recovered under this chapter, the body politic or corporate liable for such damages may after the right to such damages has become vested effect such settlement of the damages with the person entitled thereto as it may deem to be for its best interests, and it may as one of the terms of the settlement of a claim for damages growing out of the proceedings in respect of which betterments have been or are to be assessed agree in writing with the owner of the land assessed to assume such betterments. Every settlement under this section shall be in writing and in full satisfaction of all damages for such taking with interest thereon and taxable costs, if any. After approval of such settlement by the board of officers who made the taking for which damages are sought, or after acceptance of an offer of such settlement by the person seeking such damages, whichever occurs later, the amount of such settlement shall bear interest at the rate calculated pursuant to the provisions of section thirty-seven from the date of such settlement to and including the last day of the month prior to the month in which payment thereof is made or tendered.
Mass. Gen. Laws ch. 79, § 39