“(7) Where the property which the purchaser is required to dispose of in pursuance of the purchase notice has since the original disposal increased in monetary value owing to any change in circumstances (other than a change in the value of money), the amount of the consideration payable to the purchaser for the disposal by him of the property in pursuance of the purchase notice shall be the amount that might reasonably have been obtained on a corresponding disposal made on the open market at the time of the original disposal if the change in circumstances had already taken place.”