Mass. Gen. Laws ch. 35 § 10
The treasurer shall collect, receive and safely keep all money belonging to the county, and disburse it according to law. He shall not make payments to the county commissioners or associate commissioners to be disbursed by them in behalf of the county. He shall pay all claims against his county within thirty days after their allowance. He may pay any claim by a bank check which, when paid and returned, shall be a sufficient receipt therefor.
All monies held in the name of a county or a county retirement system, or any other account under the jurisdiction of a county government, or by a county officer, which are not required to be kept liquid for purposes of distribution for the thirty-day period next following, shall be invested in such a manner as to require the payment of interest on the money at the highest possible rate reasonably available. It shall be the fiduciary duty of all officers of a county government who control the investment of such funds to invest them prudently so as to accrue the highest amount of interest reasonably available on such funds.
No officer of a county shall accept a loan or other thing of value from any institution or business where he has, as a part of his official duties, invested or committed county funds.
Mass. Gen. Laws ch. 35, § 10