Mass. Gen. Laws ch. 244, § 7

If, after an execution on a judgment for possession has been levied, the amount due on the mortgage and costs are paid in full, the mortgagee, his executor, administrator or assigns shall, at the expense of the mortgagor, enter on the margin of the record of the execution an acknowledgment of satisfaction or make to the mortgagor a deed of release, which shall be recorded with notes of reference to the execution discharged thereby.