Massachusetts General Laws Part III. Courts, Judicial Officers and Proceedings in Civil Cases (Ch. 211-262) Ch. 254, § 17

A claim due absolutely and without condition, although not payable at the time of determination, shall be allowed with a rebate of interest to the time when it would become payable. If the owner has failed to perform his part of the contract and by reason of such failure the other party is, without his own default, prevented from completely performing his part thereof, he shall be entitled to a reasonable compensation for as much as he has performed, in proportion to the price stipulated for the whole.