Every county convention shall have the power to establish salaries, benefits and other compensation paid to elected county officers including the county attorney, sheriff, register of deeds, treasurer, and county commissioners. For the purposes of this section, “compensation” shall include salary, longevity pay, vacation and sick pay, allowances, and all other payments made by the county to its officers, plus the fair market value of any compensation paid in kind if reportable as income for federal income tax purposes, plus all fringe benefits that may be provided including health insurance and retirement, and may also include an upper limit on the amount of mileage and out-of-pocket expenditures reimbursable to each officer. Said compensation shall be established biennially by the county convention prior to the filing date required under N.H. Rev. Stat. § 655:14 for the elected offices listed in this section, upon recommendation of the executive committee which shall remain in effect during their term of office. Notwithstanding any other provision of law to the contrary, in counties in which any of the officers listed in this section receive fees or mileage, or both, for services performed by them as part of their compensation, the county convention may put such officer on a salary and expenses basis. Such officer may be required to continue to collect the usual fees and mileage for the service performed and to pay over all such fees and mileage to the county treasurer for the use of the county. In such event, the amount such officer received in fees and mileage, less expenses, shall be included in determining the minimum at which his salary may be established unless a lesser amount is agreed upon by the incumbent officer at that time. In no case shall the salary or other compensation of any of such officers be established at a lesser amount than that which was in effect December 31, 1972.