The tax imposed by this chapter shall be a lien upon all gifts made during the calendar year, for ten years from the time the gifts are made. If the tax is not paid when due, the donee of any gift shall be personally liable for such tax to the extent of the value of such gift. Any part of the property comprised in the gift sold by the donee to a bona fide purchaser for an adequate and full consideration in money or money's worth shall be divested of the lien herein imposed and the lien, to the extent of the value of such gift, shall attach to all the property of the donee (including after-acquired property) except any part sold to a bona fide purchaser for an adequate and full consideration in money or money's worth. If the Director is satisfied that the tax liability has been fully discharged or provided for, he may under regulations prescribed by him issue his certificate, releasing any or all of the property from the lien herein imposed.