59. Where the principal money secured is one hundred taka or upwards, a mortgage other than a mortgage by deposit of title-deeds can be effected only by a registered instrument signed by the mortgagor and attested by at least two witnesses ²[: Provided that a mortgage by deposit of title-deeds, where the mortgagee is the Government or a Scheduled Bank as defined in the Bangladesh Bank Order, 1972 or a financial institution as defined in the Financial Institutions Act, 1993, shall also be effected only by a registered instrument in the aforesaid manner ³[: Provided further that where in pursuance of a tripartite agreement between a buyer, a seller and a Scheduled Bank or financial institution, the buyer undertakes to buy a flat or floor space to be constructed or a plot of land to be developed by the seller (a real estate owner, firm or company), and the seller undertakes to transfer the flat or floor space or a plot of land, as the case may be, to the buyer by registered instrument upon construction or development, as the case may be, and the Scheduled Bank or financial institution undertakes to pay the price, or part of the price, of the flat or floor space or plot of land to the seller on behalf of the buyer, on condition, among others, that the flat or floor space to be constructed or the plot of land to be developed, and transferred to the buyer, shall be mortgaged to the Scheduled Bank or financial institution by deposit of title-deed as security against the money paid or to be paid to the seller on behalf of the buyer as loan, such mortgage by deposit of title-deed shall not be required to be effected by a registered instrument as aforesaid.]] Where the principal money secured is less than one hundred taka, a mortgage may be effected either by a registered instrument signed and attested as aforesaid, or (except in the case of a simple mortgage) by delivery of the property.