(a) Undue Hardship. “Undue hardship” as used in this connection means more than an inconvenience to the estate. It must appear that:

If a market exists, a sale of property at the current market price is not ordinarily considered as resulting in an undue hardship. The necessity for selling an interest in a qualified family property (see Revenue and Taxation Code Section 14143.5), which is included in the estate, to unrelated persons will be considered to be an undue hardship even though the interest could be sold at a price equal to its current fair market value.

(b) Adequate Security. The Controller must find that payment of the tax plus interest due thereon is adequately secured before entering into any agreement respecting its payment.

(c) Terms of Agreement. The terms of any such agreement shall provide that the executor, administrator or trustee in addition to the transferee or beneficiary shall remain personally liable for the tax plus interest due thereon until paid. The agreement shall provide for payment in no more than 10 annual installments. The first installment, together with any interest due, shall be paid on or before a date specified in the agreement. Each subsequent installment shall be in an amount not less than 10 percent of the tax imposed plus the interest computed on the unpaid balance, and it shall be paid on or before the anniversary date of the first installment payment. If the qualified family property is transferred to any person who would not qualify for the extension of time to pay inheritance tax without first obtaining a release pursuant to Revenue and Taxation Code Section 14308, the unpaid balance of tax due plus accrued interest shall become payable immediately upon such transfer.