If a bond or other obligation for the payment of money is acquired as an investment for any such common trust fund at a cost in excess of the par or maturity value thereof, the city may during but not beyond the period that such obligation is held as an investment in such fund, amortize such excess cost out of the income on such obligation by deducting from each payment of income and adding to principal an amount equal to the sum obtained by dividing such excess cost by the number of periodic payments of income to accrue on such obligation from the date of such acquisition until its maturity date.
53 P.S. § 5506