If a tax has been paid on a gift of property subject to a contingent encumbrance or contingency but valued without allowance therefor, (see Section 15556), and upon the taking effect of the encumbrance or the happening of the contingency the interest of the donee in the property becomes abridged or diminished, the person who paid the tax, or his heir, the executor of his will, or the administrator of his estate, but not his assign, is entitled to a refund in an amount equal to the difference between the tax paid and the tax which would have been paid on a valuation of the donee's interest in the property after the taking effect of the encumbrance or the happening of the contingency An application for the refund must be filed with the Controller within six months after the taking effect of the encumbrance or the happening of the contingency.

Note: Reference: Section 16223, Revenue and Taxation Code.