Section 55. Except as otherwise expressly provided, all salaries established under the authority of sections forty-eight to fifty-six, inclusive, and the salaries established by section ninety-four of chapter two hundred and twenty-one, shall be in full compensation for all services rendered, and every officer or employee shall pay all fees and other moneys received directly or indirectly in the course of his public employment, into the county treasury; provided, that nothing contained in said sections shall prevent the reimbursement of actual and other expenses necessary for the transaction of public business or the payment of overtime approved by the county commissioners, upon evidence of necessity therefor.