SECTION 3. Credits on transfers of future interests

If after the payment of the tax under this chapter there shall become payable a tax under chapter sixty-five upon any future interest in any property owned by such decedent or subject to such tax as a part of or in connection with his estate, such part of the tax paid under this chapter as is attributable to such future interest shall be credited against the tax under said chapter sixty-five, but the amount so credited shall not in any event exceed the amount of the tax on such future interest.