N.J. Stat. § 30:8-25
The board of chosen freeholders of any county in this state may cause to be employed within such county any or all prisoners in any county jail under sentence, or committed for nonpayment of a fine and costs, or committed in default of bond for nonsupport of family.
When prisoners are so employed they shall be in the custody and control of the sheriff, who shall make rules and regulations governing their employment. The sheriff may provide necessary guards and attendants while prisoners are so employed and fix their duties and salaries, as well as the terms of their employment.
The wages and salaries of such guards and attendants, and other necessary expenses in connection therewith, together with wages or earnings paid to prisoners as hereinafter provided, shall be included in and shall be charged against the cost of such work.
N.J.S. § 30:8-25