Expenditures shall be made solely in accordance with the budget, and should revenues accrue at a rate below the anticipated amounts, the board of park and recreation commissioners shall reduce expenditures accordingly: PROVIDED, That the board may, by unanimous vote, authorize such expenditures, or authorize expenditures in excess of those budgeted, if sufficient revenue to pay such expenditures is derived by the levy of the district or if provided by other governmental agencies specifically for such purposes.