Buyer took a signed agreement with the owner of the house

We were approached by a agent in the month of April,2017 who was willing to help us in selling our home in delhi and to charges an appropriate commission if the sale price is above 80 lakhs. Last week he told us that one buyer is interested and he would be coming over to our home for the token money of 10 lakh rupees. As promised he came with one person and a driver and later buyer came who were like 6 people. They pressurized my mother to sign a agreement and gave that 10 lakh cash to the agent. Who counted and agreed with the payment received. Agent told my mother that they wanted to see the ground floor once more and told he would go to notary for stamping purposes. My mother told him to wait but while showing the house the agent ran away with the money. Buyer left during that time and did not provide any copy of the agreement which my mother signed. we have the original registry with us. We have filed the compliant on the same day but the police did not file the FIR till now. Police have told us to provide the complaint copy to the registrar office at the earliest to consider any agreement signed null and void. Please help with your suggestions as I am worried as we don't know whether the buyer was involved in this cheating or not or what they have taken in the signed agreement.