Area mentioned in ares

I baught an a plot in a agricultural land, seller has mentioned land area in ares instead of square feet. Now we have found a big mistake that he converted the area in ares incorrectly and its commng only 40square feet instead of 4050 suare feet. So my questions here are 1) how many ares for 4050 square feet 2) He is not accepting the mistake so what should be our next step 3) Before making documents he said that writing land area in square feet will attract more stamp duty/registration charges than area in ares, is it true? Guys please help, I am feeling very depressed that i have been duped with all the money i had.