Mass. Gen. Laws ch. 127, § 144

A prisoner confined in a prison or place of confinement for non-payment of a fine or a fine and expenses shall be given a credit of $90 on such fine or fine and expenses for each day during which he shall be so confined, and shall be discharged at such time as the said credits, or such credits as have been given and money paid in addition thereto, shall equal the amount of the fine or the fine and expenses; and in such case no further action shall be taken to enforce payment of said fine or fine and expenses.