Under which circumstances does a contract require prior approval from the Central Government in India as per section 12 of the Airport Authority of India Act?

Section 21.   Mode of executing contracts on behalf of the Authority: (1) Every contract shall, on behalf of the Authority, be made by the Chairperson or such other member or such officer of the Authority as may be generally or specially empowered in this behalf by the Authority and such contracts or class of contracts as may be specified in the regulations shall be sealed with the common seal of the Authority:Provided that no contract exceeding such value or amount as the Central Government may, from time to time, by order, fix in this behalf shall be made unless it has been previously approved by the Authority:Provided further that no contract for the acquisition or sale of immovable property or for the lease of any such property for a term exceeding thirty years and no other contract exceeding such value or amount as the Central Government may, from time to time, by order, fix in this behalf shall be made unless it has been previously approved by the Central Government.