Cancel the deal after taking advance and refunding

We have a property in Hyderabad and unfortunately we lost the original doc of the property which was owned by us from 22yrs. We did required steps to avail the true copy of the original. But people are skeptical to buy the property. Recently getting tired selling this property, my mom spoke to someone near by my home and taking advantage of the situation he managed to convince my mom for very less money and gave a advance of around 1lakh. As me and my brother were not happy with the deal asked her to cancel it and refund his amount the next day, for which he is not agreeing to increase the sale amount nor taking back his money. At the time of taking advance my mom has signed a document on regular paper which had title as agreement/receipt and printed material which does not talk about what will happen if not paid remaining balance of the date specified. Details in the paper are filled in opposite way I.e. as if my mom is giving money to him for selling our plot and she signing it on revenue stamp and there is no sign of the buying party but has the witness signatures. Please let me know how can we legally make him take back his refund and also he has local power and is warning us he is going to cause problems if we sell to someone else. Please help us.