First and foremost thing I would like to ask you is that why didn’t you check the measurement of property at the time of buying it. Usually it happens that when you are going to buy a property you take the measurement before making any kind of payment, but as you have made the blunder of not acting in accordance with rule, certainly you deserve at least the mental agony of not getting exactly what you would have got.
But you have been cheated for sure, so there are some remedial actions also that you can take. The first thing you should do is that meet the person in flesh and blood and maintaining a professional demeanour you should say firmly that the person has cheated him and the consequence can be disastrous for his reputation. If you do that you can expect that at least the person will return the money to you that he has taken for extra money. But if you talk of the law, you have to meet some attorney who can suggest you the ways to get your money back.
Who gave you the original information? Did you obtain a survey? Look at the original purchase offer and see what was listed for the size of the property. If you were mislead or lied to, you may have a legal recourse.