The Department of General Services shall keep a record of all expenditures chargeable against each specific portion of the revolving fund, and any unencumbered balance in any portion of the fund, either within three months after completion of the project for which the portion was transferred or within three years from the time the portion was transferred or deposited therein, whichever is the earlier, shall be withdrawn from the revolving fund and transferred to the credit of the fund from which the appropriation was made or, as to funds from other than state appropriations, be paid out or refunded as provided in the agreement relating to the
contributions; provided, that on approval of the Department of Finance the time of the withdrawal may be extended.
For the purpose of this section an estimate, prepared by the Department of General Services upon receipt of bids, of the amount required for supervision, engineering, and other items, if any, necessary for the completion of a project on which a construction contract has been awarded shall be deemed a valid encumbrance and be included with any other valid encumbrances in determining the amount of an unencumbered balance.