(b) When Federal Extension Granted Upon Application - Whenever an individual, upon application, is granted a Federal extension for filing his U.S. income tax return, such extension shall not affect the date upon which the individual's Vermont income tax return is required to be filed unless a copy of the Federal extension granted by the Internal Revenue Service is filed at the main office of the Vermont Department of Taxes on or before the date the U.S. income tax return is required to be filed for that year (without regard to any extension of time for filing that return). Whenever a copy of a Federal extension is so filed, that individual shall be granted an automatic extension for the filing of his Vermont income tax return until such time as the Federal extension expires or is otherwise terminated: Provided, however, that when the Commissioner is satisfied that the individual's Vermont income tax liability has not been fully discharged by withholding or installment payments, he may, as a condition to the granting of a Vermont extension, require the payment of the amount which in his opinion is necessary to discharge fully the individual's Vermont income tax liability.