Mass. Gen. Laws ch. 200A § 13
All agreements to pay compensation to recover or assist in the recovery of property reported under section seven made within twenty-four months after the date payment or delivery is made under section eight A are unenforceable. Such an agreement made more than twenty-four months after payment or delivery is required under said section eight A is valid if: (a) it is in writing, signed by the owner and discloses the nature and value of the property and the name and address of the person or entity in possession of the property. (b) the fee or compensation does not exceed ten percent at any time.
Nothing in this section shall be construed to prevent an owner from asserting, at any time, that any agreement to locate property is based on excessive or unjust compensation.
Mass. Gen. Laws ch. 200A, § 13