Where a guardian has been appointed by will or other instrument, his power to mortgage or charge, or transfer by sale, gift, exchange or otherwise, immovable property belonging to his ward is subject to any restriction which may be imposed by the instrument, unless he has under this Act been declared guardian and the Court which made die declaration permits him by an order in writing, notwithstanding the restriction, to dispose of any immovable property specified in the order in a manner permitted by the order.