Where such accession has been acquired at the expense of the mortgagee, and is capable of separate possession or enjoy-ment without detriment to the principal property, the mortgagor desiring to take the accession must pay to the mortgagee the expense of acquiring it. If such separate possession or enjoyment is not possible, the accession must be delivered with the property; the mortgagor being liable, in the case of an acquisition necessary to preserve the property from destruction, forfeiture or sale, or made with his assent, to pay the proper cost thereof, as an addition to the principal money, with interest at the same rate as is payable on the principal, or, where no such rate is fixed, at the rate of nine percent. per annum.In the case last mentioned the profits, if any, arising from the accession shall be credited to the mortgagor.Where the mortgage is usufructuary and the accession has been acquired at the expense of the mortgagee, the profits, if any, arising from the accession shall, in the absence of a contract to the contrary, be set off against interest, if any, payable on the money so expended.