Seller claiming more than the agreed upon price

A certain person owned a property that his friend wished to buy from him. On seeing the land, his friend offered him R1.2 million. He told his friend, “I don’t know the value of the property. If the value is more, you must sort me out.” After a week passed, the owner still hadn’t checked the value, but concluded a deal for R1.2 million, even making out a contract and signing the papers. A few months later, the initial owner learnt that the value was R1.5 million. He now wants to claim an additional R300 000 from the seller. Is this permissible?