Any balance in the overlay account of a given year in excess of the amount remaining to be collected or abated can be transferred into this account. Within ten days of a written request by the chief executive officer of a city or town, the assessors must provide a certification of the excess amount of overlay available to transfer. Overlay surplus may be appropriated for any lawful purpose. At the end of each fiscal year, unused overlay surplus is "closed" to surplus revenue; in other words, it becomes a part of free cash.