If the inheritance tax is not paid before the date it becomes delinquent, interest on the unpaid tax shall be charged after the date of delinquency at the rate established pursuant to section 806 of the act of April 9, 1929 (P.L. 343, No. 176),¹ known as “The Fiscal Code.” When payment of inheritance tax is not made because of litigation or other unavoidable cause of delay and the property on which the tax has been calculated has remained in the hands of a fiduciary and has not produced a net income equal to the rate of interest provided in this section annually, interest for such period shall be calculated at the rate of the net income produced by the property. Any payment on delinquent inheritance tax shall be applied first to any interest due on the tax at the date of payment and then, if there is any balance, to the tax itself.