Section 36A. Motor vehicles to be purchased from sums appropriated by the general court shall be authorized by the purchasing agent in accordance with schedules filed by the budget director with the house and senate committees on ways and means of the general court prior to the passage of the act making such appropriation provided, the commissioner of administration may authorize the replacement of other motor vehicles with similar models from available funds when he determines that the replacement is necessary because the cost of necessary repairs would not be economical; and provided further, that no expenditure in excess of fifteen hundred dollars shall be authorized for the repair of a motor vehicle whose purchase price is fifteen thousand dollars or less and no expenditure in excess of three thousand dollars shall be authorized for the repair of a motor vehicle whose purchase price is greater than fifteen thousand dollars by any department or agency in a garage or shop other than one maintained and operated by such department or agency or by the executive office of administration and finance without the prior written approval of the commissioner of administration.