Mass. Gen. Laws ch. 106, § 2-202: Final Written Expression: Parol or Extrinsic Evidence

Section 2–202. Terms with respect to which the confirmatory memoranda of the parties agree or which are otherwise set forth in a writing intended by the parties as a final expression of their agreement with respect to such terms as are included therein may not be contradicted by evidence of any prior agreement or of a contemporaneous oral agreement but may be explained or supplemented

(a) by course of performance, course of dealing or usage of trade, in section 1–303; and

(b) by evidence of consistent additional terms unless the court finds the writing to have been intended also as a complete and exclusive statement of the terms of the agreement.