By virtue of paragraph(s) 3 of Part 1 of Schedule 12A of the Local Government Act 1972.
Information relating to the financial or business affairs of any particular person (including the authority holding that information).
The public interest in maintaining the exemption outweighs the public interest in disclosing the information because this is an offer to acquire a property and disclosure of the offer could result in the sale falling through and a loss of a capital receipt to the Council.