Cal. Code Regs. tit. 18, § 13956 – Contingent Encumbrances and Contingencies.

(a) In the valuation of any property in which a transferee is given a present right of enjoyment or possession, no allowance will be made on account of any contingent incumbrance on the property, nor on account of any contingency upon the happening of which the transferee's interest might, in whole or in part, be abridged, defeated, or diminished. If, however, the incumbrance or contingency should become effective after the transferee has become possessed, or has entered upon the enjoyment, of the property, and an inheritance tax has been paid, a refund of tax may be secured by the person who paid the tax.
(b) Under proper circumstances, any tax due in respect to the transfer of any property covered by this regulation may be the subject of a compromise, see Revenue and Taxation Code Sections 14191 and 14192.
Note: Reference: Section 13956, Revenue and Taxation Code.